I use LRP 3.5 with HW 120A turbo on, silverback 50 and 60C not issue wor.... what is your roll out? Maybe gear ratio issue? BTW: My car after 8min run end voltage (w/o load) is around 3.6-3.7V.
4.38 should be ok wor.... 50mm rear tire roll out 36mm. 45mm rear tire roll out 32mm.... I use 33mm roll out with 3.5motor no problem wor...... Hum...... don't know.....
I know 64pitch, but the size of the pinion/spur will affect the motor's efficiency slightly.
maybe you can try a larger spur, like a 96 or even 100.
(usually a smaller spur requires the motor to work more, thus using more current on your battery)
Also, are you using turbo/boost function on your sp2.1?