Try a 14 tooth pinion or maybe a 13. I'm using a 4500 copperhead combo. So far it seems fine...I had mid 70 temps the other day but to be fair, it was in the mid 50's.
I also have a monster mamba setup on 4s. I have the motor shut down to 60% just to keep from ripping apart the axles and drive train.

A little more info...I still have the 2 speed tranny setup and I'm using the mechanical servo operated brakes. The 4500 motor is setup for forward only and does not use the esc for braking. I believe this reduces the strain on the motor/esc.
Possibly why my temps are lower...