Can I run a Mega 2560 on 14v?
You can, but you shouldn't, because the the board will heat excessively, as you're out of the recommended range for the input voltage on vin (7-12V).

Unless you have a switching regulator (the one on the board is linear), then you shouldn't' have too much of a difference between the operating voltage and vin as an operating principle because of efficiency (and thus heat) issues.