Just got done testing the MMP. The final results are after 8 to 10 minutes of pretty aggressive driving (speed passes, lots of braking, lots of acceleration, etc):

Motor 167F
Batt 96F
ESC 150F to 165F, depending on where I aimed the temp gun
Ambient temp is 32F

Specs:
5S 40C 3000mAh Zippy LiPo
MMP with 1800kV Neu motor
No airflow provisions in the body
46T spur with 21T pinion

Also this is without a fan on the ESC. I had to remove the fan for it to fit in my RC10B4. If I decide to keep the MMP in the RC8Be, I'll order a new fan for it.

I'm charging up a 6S now to give it a try.

As for the leaking diffs, I filled mine up just past the spider gears. Some did come out when I assembled the diff (from being slightly overfilled, I assume), so I cleaned that up after I tightened the screws (I tightened them as evenly as I could). I let the diff sit overnight and no leaking. Also no leaking after a post run inspection. Before I used the Mugen gasket, the diff showed signs of weeping after I let it sit overnight.