That's the way it works. Your motors WILL get hotter if you run them twice as long. There is no way around it. Even the best motors get hot on high mah battery packs. Your solution is simply to water cool your motor(s). It's so easy to do and helps a lot.

And to answer your other question, NO you won't have to ugrade your ESC as mah has nothing to do with voltage delivered. Mah is simply a measure of how long the cells will last in milli amp hours. Like fuel in a tank, 3000 mah battery packs last twice as long as 1500 mah battery packs (assuming the same quality and peak charging).

Lastly, if you don't already have one, you should use a peak charger (like a dynamite peak charger) to get the most out of your batteries. Make sure you are fully discharging your current batteries (ni-cd's) prior to re-charging so they don't get the "memory effect". If you move to ni-mh cells this is a non issue. Good luck.