I haven't done it with a Mega2560 chip, but for the past year, I've been running the 328
chip at 16Mhz and 3.3V, and the past month been doing same with the 1284P chip. And
no problems, although against all warnings - of course. If you interpolate the operation
curves, they actually show 13.3 Mhz at 3.3V, so you're only overclocking by 20%.
Also, for hacking, the 40-pin DIP 1284P chip is a nice choice inbetween the 328 and 2560,
and lots easier than tiny smt pins. You get 32 I/O pins.