I used to run a 5700 on 3s in my rustler, and it would absolutely cook even when geared down to 25-30mph (still plenty of speed really). I could only ever get about 3-4mins running with it before the motor felt like it should be glowing red. I guess the 40C summers here didnt help with temps much
I now run a 9L on 3s geared to 35mph (now in my crt.5) and it doesnt get much above ambient, after 20min of hard starts and hard braking. I much prefer it over the sub S can castle motors. The new castle motors could be a lot better I guess.