The spur diff melts if it's too loose and slipping a lot. The temperature where I live is about 14°C when I drive, so it's not exactly hot, but there's no sign of damage on my spur gear. It slips occasionally. I can hear it if I punch it on launch. I have tightened it slightly beyond a flattened spring washer. I should really add some washers to increase the spring tension without flattening.
I run a 17,5T brushless with a 28 teeth pinion which is the biggest that would fit and all is running well.
Every time I run mine I have to remove the right rear wheel to empty out all the debris that worked its way through the gap between the wheel and hub. It's pretty odd how much crap gets in there, especially since my track is counter clockwise.