[BrianG]

What I see here is you are going from battery, to converter, to some sort of "speed control" (which does it's own converting), then to motor. Seems like there is an extra unneeded stage here. People's arguments of adding something else to the chain will lose efficiency does have merit. Why not simply do away with the converters and stock speed control and use a high current ESC in brushed mode and run that on 4s? The MMM is good for that, even in reversing brushed mode (and will have some programming features your current setup doesn't have, not to mention an LVC). And a simple ESC would weigh much less than those converters, wiring, and stock speed control, and retain greater efficiency since there are fewer stages. You can use an e-bike twisty knob thingy as the speed adjustment going to a servo tester.