Got the diff locked but still waiting for my 7 cell. I have a 15t pinion right now but I think I may need to go lower. Motor dont seem to have enough power to push the car if I am going and flip a quick 180 and try and take off. Also motor seems to get a bit warm after some hard running. Might invest in a fan kit too.
I would try another motor heatsink, since your using the motor mount that has a heat sink i would also put another on the motor........Check the picture of mine that i posted.....