Alternative methods of powering are to provide an external regulator (either linear like the on-board one, or switch-mode DC->DC converter) to provide 5V - heatsinking can then be used to deal with higher
currents (or the DC->DC converter option is less wasteful anyway, typically > 80% power efficiency).
You mean 7v (not 5v) to the barrel jack, correct? Both the Uno and Mega spec sheets now warn about supplying 5v power directly to the 5v pin. It can cause damage.
From the Uno reference page:
Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.