I run a different ESC than you, but the timing settings may be similar in general. I'd say start at the 15T pinion if the temps felt reasonable and start upping the timing slowly, watching temps.

you may even find that you will go down in pinion size with more timing.

Here is an example of the kind of timing guys were doing with a Novak 5.5 and the Speed Passion GT2.0 Pro ESC in a 4x4 SCTE:



not sure if the MMP has a turbo timing feature (timing at full throttle) but the boost timing could be similar to the CHEAT mode. Again, start slow with timing adjustment and don't be afraid to go DOWN in pinion size.