I was running Speed 300 and 15T pinions, and 8 cells of 2/3A batteries. The motor reached 200F+ when I was playing on the grass....
I used my RayTek temp gun to check the motor.
Then, my pinion melted. Only after 5 minutes hard running.
Now, I'm using WattAge 370 motor with steel 11T pinion. The motor runs much cooler around 110F. And I can do wheelie on both side while standing still....
Smaller pinion gear will help your motor running cooler for sure. but you will loose the top speed.
The clutch is to protect the drive train if you like to jump the little truck alot. I would like to keep it in the gear box. I already borke one teeth on the rear ring gear.