I have the medusa 3400 motor (and the 2800 too) and I put the 3400 on a 4 cell 10T t-rex and compared it to the 2800 4 cell setup (in a different t-rex). I like the 2800 a little better on 4 cells, it seemed like the 2800 had more torque and better throttle response, although the 3400 did pretty darned well. The 3400 did get a little warm and I'm sure it would need a heat sink here in the summer on 4 cells. I plan to try it on 3 cells where I'm sure it will be much happier. Right now the 4 cell 2800 medusa is my favorite t-rex setup.
Just for grins I put a neu 1107/2Y (also 3400 kv) with a 10T pinion on the same 4 cell t-rex as the medusa 3400/10T for a direct comparison. I really couldn't see much difference, they felt the same in the air. The neu ran a little cooler but the performance was essentially the same. I've read where these motors might be closer to their sweet spot using a 9T pinion but a 10T pinion was the smallest pinion I could find that would fit the motor shafts. YMMV.
I'm currently running the neu 1107/2Y on a 3 cell 1800 mah Evo20 battery and it does very well. I suspect the Medusa 3400/3-cell setup will be just as good based on the earlier 4 cell tests. All the newer high C rated 3 cell batts might allow the 3 cell setups to catch up, if not surpass, the 4 cell setups. It's all fun and I'm rapidly finding that I need to be a better pilot to discern the power differences as the setups get stronger and stronger. They've all gotten so good that now I'm definitely the weak link in the system.
Good luck, it's all one big game of experimentation but that's half the fun.