I just now did the experiment using a 60 byte payload on the supercap, which I had just finished charging for more than a day using the charger circuit powered by a couple of AA's (so that it could charge at night as well). Here are the early results:
101,[Sender:2],352 ABCDEFGHIJKLMNOPQRSTUVWXYZ012345678901234567890123456789,[RX_RSSI:-36]
102,[Sender:2],352,[RX_RSSI:-36]
103,[Sender:2],344,[RX_RSSI:-35]
104,[Sender:2],344,[RX_RSSI:-34]
105,[Sender:2],335,[RX_RSSI:-35]
106,[Sender:2],335,[RX_RSSI:-35]
107,[Sender:2],335,[RX_RSSI:-34]
108,[Sender:2],335,[RX_RSSI:-35]
109,[Sender:2],335,[RX_RSSI:-35]
110,[Sender:2],331,[RX_RSSI:-34]
111,[Sender:2],327,[RX_RSSI:-35]
112,[Sender:2],327,[RX_RSSI:-35]
113,[Sender:2],327,[RX_RSSI:-35]
114,[Sender:2],327,[RX_RSSI:-35]
115,[Sender:2],327,[RX_RSSI:-35]
116,[Sender:2],327,[RX_RSSI:-35]
117,[Sender:2],327,[RX_RSSI:-35]
118,[Sender:2],327,[RX_RSSI:-35]
119,[Sender:2],327,[RX_RSSI:-35]
120,[Sender:2],327,[RX_RSSI:-35]
121,[Sender:2],327,[RX_RSSI:-35]
122,[Sender:2],327,[RX_RSSI:-35]
123,[Sender:2],327,[RX_RSSI:-34]
124,[Sender:2],327,[RX_RSSI:-35]
125,[Sender:2],327,[RX_RSSI:-35]
126,[Sender:2],327,[RX_RSSI:-35]
127,[Sender:2],327,[RX_RSSI:-35]
128,[Sender:2],323,[RX_RSSI:-35]
129,[Sender:2],323,[RX_RSSI:-35]
130,[Sender:2],323,[RX_RSSI:-35]
131,[Sender:2],323,[RX_RSSI:-35]
132,[Sender:2],323,[RX_RSSI:-35]
133,[Sender:2],323,[RX_RSSI:-35]
134,[Sender:2],323,[RX_RSSI:-35]
135,[Sender:2],323,[RX_RSSI:-34]
136,[Sender:2],323,[RX_RSSI:-35]
137,[Sender:2],319,[RX_RSSI:-36]
138,[Sender:2],318,[RX_RSSI:-36]
139,[Sender:2],352 ABCDEFGHIJKLMNOPQRSTUVWXYZ012345678901234567890123456789,[RX_RSSI:-35]
140,[Sender:2],352,[RX_RSSI:-35]
141,[Sender:2],344,[RX_RSSI:-35]
142,[Sender:2],335,[RX_RSSI:-35]
143,[Sender:2],335,[RX_RSSI:-35]
144,[Sender:2],335,[RX_RSSI:-35]
145,[Sender:2],335,[RX_RSSI:-37]
146,[Sender:2],335,[RX_RSSI:-41]
147,[Sender:2],327,[RX_RSSI:-47]
148,[Sender:2],327,[RX_RSSI:-45]
149,[Sender:2],327,[RX_RSSI:-46]
150,[Sender:2],327,[RX_RSSI:-47]
151,[Sender:2],327,[RX_RSSI:-44]
152,[Sender:2],327,[RX_RSSI:-46]
153,[Sender:2],327,[RX_RSSI:-44]
154,[Sender:2],327,[RX_RSSI:-46]
155,[Sender:2],327,[RX_RSSI:-46]
156,[Sender:2],327,[RX_RSSI:-45]
157,[Sender:2],327,[RX_RSSI:-45]
158,[Sender:2],327,[RX_RSSI:-46]
159,[Sender:2],327,[RX_RSSI:-45]
160,[Sender:2],327,[RX_RSSI:-46]
161,[Sender:2],323,[RX_RSSI:-44]
162,[Sender:2],323,[RX_RSSI:-45]
163,[Sender:2],323,[RX_RSSI:-45]
164,[Sender:2],323,[RX_RSSI:-44]
165,[Sender:2],323,[RX_RSSI:-46]
166,[Sender:2],323,[RX_RSSI:-44]
167,[Sender:2],319,[RX_RSSI:-45]
168,[Sender:2],319,[RX_RSSI:-45]
169,[Sender:2],319,[RX_RSSI:-44]
170,[Sender:2],319,[RX_RSSI:-45]
171,[Sender:2],319,[RX_RSSI:-45]
172,[Sender:2],319,[RX_RSSI:-46]
173,[Sender:2],319,[RX_RSSI:-44]
174,[Sender:2],319,[RX_RSSI:-44]
175,[Sender:2],319,[RX_RSSI:-46]
176,[Sender:2],318,[RX_RSSI:-44]
177,[Sender:2],352 ABCDEFGHIJKLMNOPQRSTUVWXYZ012345678901234567890123456789,[RX_RSSI:-46]
178,[Sender:2],352,[RX_RSSI:-46]
179,[Sender:2],344,[RX_RSSI:-45]
180,[Sender:2],335,[RX_RSSI:-46]
181,[Sender:2],331,[RX_RSSI:-46]
182,[Sender:2],331,[RX_RSSI:-46]
183,[Sender:2],327,[RX_RSSI:-46]
184,[Sender:2],327,[RX_RSSI:-45]
185,[Sender:2],327,[RX_RSSI:-46]
186,[Sender:2],327,[RX_RSSI:-45]
187,[Sender:2],327,[RX_RSSI:-46]
188,[Sender:2],327,[RX_RSSI:-47]
189,[Sender:2],327,[RX_RSSI:-46]
190,[Sender:2],327,[RX_RSSI:-47]
191,[Sender:2],327,[RX_RSSI:-46]
192,[Sender:2],327,[RX_RSSI:-46]
193,[Sender:2],327,[RX_RSSI:-45]
194,[Sender:2],327,[RX_RSSI:-41]
195,[Sender:2],327,[RX_RSSI:-36]
196,[Sender:2],323,[RX_RSSI:-36]
197,[Sender:2],323,[RX_RSSI:-36]
198,[Sender:2],323,[RX_RSSI:-35]
199,[Sender:2],323,[RX_RSSI:-36]
200,[Sender:2],319,[RX_RSSI:-37]
201,[Sender:2],319,[RX_RSSI:-37]
202,[Sender:2],319,[RX_RSSI:-36]
203,[Sender:2],319,[RX_RSSI:-37]
204,[Sender:2],319,[RX_RSSI:-36]
205,[Sender:2],319,[RX_RSSI:-37]
206,[Sender:2],319,[RX_RSSI:-36]
207,[Sender:2],319,[RX_RSSI:-36]
208,[Sender:2],319,[RX_RSSI:-36]
209,[Sender:2],319,[RX_RSSI:-36]
210,[Sender:2],319,[RX_RSSI:-36]
211,[Sender:2],319,[RX_RSSI:-36]
212,[Sender:2],319,[RX_RSSI:-36]
213,[Sender:2],319,[RX_RSSI:-36]
214,[Sender:2],316,[RX_RSSI:-36]
215,[Sender:2],352 ABCDEFGHIJKLMNOPQRSTUVWXYZ012345678901234567890123456789,[RX_RSSI:-36]
216,[Sender:2],352,[RX_RSSI:-36]
217,[Sender:2],344,[RX_RSSI:-35]
218,[Sender:2],335,[RX_RSSI:-41]
219,[Sender:2],327,[RX_RSSI:-44]
220,[Sender:2],327,[RX_RSSI:-40]
221,[Sender:2],327,[RX_RSSI:-36]
222,[Sender:2],327,[RX_RSSI:-36]
223,[Sender:2],327,[RX_RSSI:-36]
224,[Sender:2],327,[RX_RSSI:-36]
225,[Sender:2],327,[RX_RSSI:-37]
226,[Sender:2],327,[RX_RSSI:-36]
227,[Sender:2],327,[RX_RSSI:-36]
228,[Sender:2],327,[RX_RSSI:-36]
229,[Sender:2],323,[RX_RSSI:-36]
230,[Sender:2],323,[RX_RSSI:-36]
231,[Sender:2],323,[RX_RSSI:-35]
232,[Sender:2],323,[RX_RSSI:-36]
233,[Sender:2],323,[RX_RSSI:-36]
234,[Sender:2],319,[RX_RSSI:-36]
235,[Sender:2],319,[RX_RSSI:-36]
236,[Sender:2],319,[RX_RSSI:-37]
237,[Sender:2],319,[RX_RSSI:-37]
238,[Sender:2],319,[RX_RSSI:-35]
239,[Sender:2],319,[RX_RSSI:-35]
240,[Sender:2],319,[RX_RSSI:-35]
241,[Sender:2],319,[RX_RSSI:-35]
242,[Sender:2],319,[RX_RSSI:-35]
243,[Sender:2],319,[RX_RSSI:-35]
244,[Sender:2],319,[RX_RSSI:-35]
245,[Sender:2],319,[RX_RSSI:-35]
246,[Sender:2],319,[RX_RSSI:-35]
247,[Sender:2],319,[RX_RSSI:-35]
248,[Sender:2],319,[RX_RSSI:-35]
249,[Sender:2],319,[RX_RSSI:-35]
250,[Sender:2],319,[RX_RSSI:-35]
251,[Sender:2],316,[RX_RSSI:-35]
252,[Sender:2],352 ABCDEFGHIJKLMNOPQRSTUVWXYZ012345678901234567890123456789,[RX_RSSI:-35]
253,[Sender:2],352,[RX_RSSI:-35]
254,[Sender:2],344,[RX_RSSI:-35]
255,[Sender:2],335,[RX_RSSI:-38]
256,[Sender:2],327,[RX_RSSI:-44]
257,[Sender:2],327,[RX_RSSI:-36]
258,[Sender:2],327,[RX_RSSI:-35]
259,[Sender:2],327,[RX_RSSI:-34]
260,[Sender:2],327,[RX_RSSI:-35]
261,[Sender:2],327,[RX_RSSI:-34]
262,[Sender:2],327,[RX_RSSI:-34]
263,[Sender:2],327,[RX_RSSI:-35]
264,[Sender:2],327,[RX_RSSI:-35]
265,[Sender:2],327,[RX_RSSI:-35]
266,[Sender:2],323,[RX_RSSI:-35]
267,[Sender:2],323,[RX_RSSI:-35]
268,[Sender:2],323,[RX_RSSI:-35]
269,[Sender:2],323,[RX_RSSI:-35]
270,[Sender:2],319,[RX_RSSI:-35]
271,[Sender:2],319,[RX_RSSI:-35]
272,[Sender:2],319,[RX_RSSI:-35]
273,[Sender:2],319,[RX_RSSI:-35]
274,[Sender:2],319,[RX_RSSI:-35]
275,[Sender:2],319,[RX_RSSI:-35]
276,[Sender:2],319,[RX_RSSI:-35]
277,[Sender:2],319,[RX_RSSI:-35]
278,[Sender:2],319,[RX_RSSI:-35]
279,[Sender:2],319,[RX_RSSI:-35]
280,[Sender:2],319,[RX_RSSI:-34]
281,[Sender:2],319,[RX_RSSI:-34]
282,[Sender:2],319,[RX_RSSI:-35]
283,[Sender:2],319,[RX_RSSI:-36]
284,[Sender:2],319,[RX_RSSI:-35]
285,[Sender:2],319,[RX_RSSI:-35]
286,[Sender:2],319,[RX_RSSI:-35]
287,[Sender:2],319,[RX_RSSI:-36]
288,[Sender:2],313,[RX_RSSI:-35]
289,[Sender:2],352 ABCDEFGHIJKLMNOPQRSTUVWXYZ012345678901234567890123456789,[RX_RSSI:-35]
290,[Sender:2],352,[RX_RSSI:-35]
291,[Sender:2],344,[RX_RSSI:-36]
292,[Sender:2],335,[RX_RSSI:-35]
293,[Sender:2],327,[RX_RSSI:-35]
294,[Sender:2],327,[RX_RSSI:-36]
295,[Sender:2],327,[RX_RSSI:-35]
296,[Sender:2],327,[RX_RSSI:-35]
297,[Sender:2],327,[RX_RSSI:-35]
298,[Sender:2],327,[RX_RSSI:-35]
299,[Sender:2],327,[RX_RSSI:-35]
300,[Sender:2],323,[RX_RSSI:-34]
301,[Sender:2],323,[RX_RSSI:-35]
302,[Sender:2],323,[RX_RSSI:-35]
303,[Sender:2],323,[RX_RSSI:-35]
304,[Sender:2],319,[RX_RSSI:-36]
305,[Sender:2],319,[RX_RSSI:-35]
306,[Sender:2],319,[RX_RSSI:-35]
307,[Sender:2],319,[RX_RSSI:-35]
308,[Sender:2],319,[RX_RSSI:-35]
309,[Sender:2],319,[RX_RSSI:-36]
310,[Sender:2],319,[RX_RSSI:-35]
311,[Sender:2],319,[RX_RSSI:-36]
312,[Sender:2],319,[RX_RSSI:-36]
313,[Sender:2],319,[RX_RSSI:-35]
314,[Sender:2],319,[RX_RSSI:-36]
315,[Sender:2],319,[RX_RSSI:-36]
316,[Sender:2],319,[RX_RSSI:-36]
317,[Sender:2],319,[RX_RSSI:-36]
318,[Sender:2],319,[RX_RSSI:-36]
319,[Sender:2],319,[RX_RSSI:-36]
320,[Sender:2],319,[RX_RSSI:-36]
321,[Sender:2],319,[RX_RSSI:-36]
322,[Sender:2],319,[RX_RSSI:-36]
323,[Sender:2],319,[RX_RSSI:-36]
324,[Sender:2],319,[RX_RSSI:-36]
325,[Sender:2],319,[RX_RSSI:-36]
326,[Sender:2],313,[RX_RSSI:-35]
327,[Sender:2],352 ABCDEFGHIJKLMNOPQRSTUVWXYZ012345678901234567890123456789,[RX_RSSI:-36]
328,[Sender:2],352,[RX_RSSI:-36]
329,[Sender:2],336,[RX_RSSI:-36]
330,[Sender:2],335,[RX_RSSI:-36]
331,[Sender:2],327,[RX_RSSI:-35]
332,[Sender:2],327,[RX_RSSI:-35]
333,[Sender:2],327,[RX_RSSI:-35]
334,[Sender:2],327,[RX_RSSI:-36]
335,[Sender:2],327,[RX_RSSI:-36]
336,[Sender:2],323,[RX_RSSI:-34]
337,[Sender:2],323,[RX_RSSI:-35]
338,[Sender:2],323,[RX_RSSI:-35]
339,[Sender:2],323,[RX_RSSI:-35]
340,[Sender:2],323,[RX_RSSI:-34]
341,[Sender:2],319,[RX_RSSI:-35]
342,[Sender:2],319,[RX_RSSI:-35]
343,[Sender:2],319,[RX_RSSI:-36]
344,[Sender:2],319,[RX_RSSI:-37]
345,[Sender:2],319,[RX_RSSI:-36]
346,[Sender:2],319,[RX_RSSI:-36]
347,[Sender:2],319,[RX_RSSI:-36]
348,[Sender:2],319,[RX_RSSI:-35]
349,[Sender:2],319,[RX_RSSI:-35]
350,[Sender:2],319,[RX_RSSI:-35]
351,[Sender:2],319,[RX_RSSI:-36]
352,[Sender:2],319,[RX_RSSI:-36]
353,[Sender:2],319,[RX_RSSI:-36]
354,[Sender:2],319,[RX_RSSI:-36]
355,[Sender:2],319,[RX_RSSI:-35]
356,[Sender:2],319,[RX_RSSI:-36]
357,[Sender:2],319,[RX_RSSI:-36]
358,[Sender:2],319,[RX_RSSI:-35]
359,[Sender:2],319,[RX_RSSI:-35]
360,[Sender:2],319,[RX_RSSI:-36]
361,[Sender:2],319,[RX_RSSI:-35]
362,[Sender:2],319,[RX_RSSI:-35]
363,[Sender:2],319,[RX_RSSI:-35]
364,[Sender:2],313,[RX_RSSI:-35]
365,[Sender:2],352 ABCDEFGHIJKLMNOPQRSTUVWXYZ012345678901234567890123456789,[RX_RSSI:-35]
366,[Sender:2],352,[RX_RSSI:-35]
367,[Sender:2],335,[RX_RSSI:-35]
368,[Sender:2],335,[RX_RSSI:-35]
369,[Sender:2],327,[RX_RSSI:-35]
370,[Sender:2],327,[RX_RSSI:-35]
371,[Sender:2],327,[RX_RSSI:-36]
372,[Sender:2],327,[RX_RSSI:-37]
373,[Sender:2],323,[RX_RSSI:-36]
374,[Sender:2],323,[RX_RSSI:-37]
375,[Sender:2],323,[RX_RSSI:-36]
376,[Sender:2],323,[RX_RSSI:-34]
377,[Sender:2],319,[RX_RSSI:-37]
378,[Sender:2],319,[RX_RSSI:-35]
379,[Sender:2],319,[RX_RSSI:-35]
380,[Sender:2],319,[RX_RSSI:-34]
381,[Sender:2],319,[RX_RSSI:-36]
382,[Sender:2],319,[RX_RSSI:-36]
383,[Sender:2],319,[RX_RSSI:-37]
384,[Sender:2],319,[RX_RSSI:-36]
385,[Sender:2],319,[RX_RSSI:-36]
386,[Sender:2],319,[RX_RSSI:-36]
387,[Sender:2],319,[RX_RSSI:-36]
388,[Sender:2],319,[RX_RSSI:-35]
389,[Sender:2],319,[RX_RSSI:-36]
390,[Sender:2],319,[RX_RSSI:-35]
391,[Sender:2],319,[RX_RSSI:-34]
392,[Sender:2],319,[RX_RSSI:-36]
393,[Sender:2],319,[RX_RSSI:-36]
394,[Sender:2],319,[RX_RSSI:-35]
395,[Sender:2],319,[RX_RSSI:-36]
396,[Sender:2],319,[RX_RSSI:-36]
397,[Sender:2],319,[RX_RSSI:-36]
398,[Sender:2],319,[RX_RSSI:-35]
399,[Sender:2],319,[RX_RSSI:-36]
400,[Sender:2],319,[RX_RSSI:-36]
401,[Sender:2],319,[RX_RSSI:-36]
402,[Sender:2],313,[RX_RSSI:-36]
As before, the very first and the very last voltage measurement in the series is a double-match. The ones in-between are all single-shot.
Here's another piece of data that may (?) have relevance: the time needed to purely Tx the 60 byte payload (not counting the time it took to load the payload into the FIFO, but purely the Tx part) was about 2160uSec, or a little bit more than twice as long as for a 3 byte payload. That is to say, with the FIFO already loaded, it takes 2160uSec from the moment the command is given to the radio to enter Tx mode to the time the PacketSent flag is detected. That is the same interval over which all of the single-shot voltages are measured. So, a bit more than twice as long to send a 60 byte payload than a 3 byte payload, even though the 6 byte payload contains 20x as many bytes. Obviously, the frame size itself is bigger than the payload itself, but apparently the RFM69 takes quite a while (relatively speaking, given that the bitrate is 300kbps) to spin up and be ready to Tx.