Sorry that I'm a little late to the party, not been around here for a while....
I've been running mega328 and 1284 chips at Vcc=3.3V, and with 16-MHz xtal for 5-years now, and never had a problem, although I do not operate at temperature extremes. OTOH, I just installed a Moteino-Mega inside my van to use in an alarm system (see related post to appear hereabouts), so I will see whether any problems creep in this winter, at temperatures below 20-degF.
With respect to the d/s, if you look very closely, you'll see that the curve crosses near 13-MHz oscillator frequency for Vcc=3.3V, and is well above 8-MHz. So, all in all, 13-MHz is only 20% distance from 16-MHz, and not 2X as most people round it to be. I should expect that a 20% tolerance is nowheres near a problem.