RE: gearing question
You can't go up nine teeth on the pinion and expect to get a result. Get a 20t pinion and see where it goes. What is the final drive ratio of the car? Did temperatures increase with the 26t pinion? What is the stock gearing?
2500kv is really low for a 13.5, and very very low for trying to get speed out of a car on 2S LiPo/6 cell NiMh.