Yup, adding a heatsink is quite easy on the Quarks. The FETs are attached to the bottom where it is flat. There are tons of ways to do this, here's one (mine :)). Believe it or not, this helped a lot. The ESC case is kind of thin (presumably to keep weight down) so it gets hot quick and there isn't much surface area to dissipate the heat. Add mass to absorb heat, then add surface area to get rid of it:
