On something large like this, just about any drop in efficiency results in a lot of wasted heat. For example, let's say you have a setup that draws 350w average with 2000w peaks:

A 75% efficient motor will eat up 87.5w cont, and 500w peak power.
An 85% efficient motor will eat up 52.5w cont and 300w peak power.

That lost power is less power being put to the ground, lots of added motor heat.

I'd say just pony up for a more suitable motor. A Neu 1515, Castle 1800 (not out yet), or a Medusa 36-80-2000 on 6s would be better choices.