Which of the stock settings did you find problematic? Sounds like you got an improvement from increasing the rxbw. I also do that at very low bitrates where the frequency offset is large relative to fdev. Did you get a benefit at 55kbit? Were the nodes at dramatically different temperatures? Anything else?
I suspect that the rxbw 'stock' settings are a little tight across the board, I've had general minor unreliability issues which, like many others, I've been assuming were RF interference issues. I'm now largely convinced that this is not the case . . . increasing rxbw has made a dramatic difference to reliability. To be clear I'm talking 0.5-2% errors . . . but that is symptomatic of an issue . . . increasing rxbw seems to remove the errors.
Secondly, I was trying to use a cluster if RFM69HW's at low OTA bitrate and ran into random issues with one node being able to receive from its peers but its peers not being able to receive it's response . . . this is the classic frequency mismatch issue I think - but all nodes were at the 'same' temp in the 'same' environment, but that environment changed temp during the day . . . in the afternoons when it was warm, problems appeared, when cooler, everything worked. So I think 'different' temps is not the only issue . . . this change has solved that problem (so far) also. I've also incorporated reasonably frequent RC osc calibration into the general scheme of things at both ends . . .
The nice part about a highly redundant Fec scheme is that you can run at high bitrates so the freq offset won't matter much. Narrow-band transmission is kind of the opposite of what I thought was your original plan.
It seems that the issue is most prevalant at low OTA bitrates, I'm looking at low bitrates for range, and FEC for additional range, my application doesn't have large payloads, trying to squeeze everything into 16bytes so it can be AES256 encrypted in one block . . .
I think minimising the rxbw is a good thing, but not so much as it causes problems . . . I have a feeling that the data sheet figures are optimal and the real world needs a little more slop ;-)