@sikes: Yeah, your setup does pose a few challenges given the wireless thing. But hey, just use an actual R/C radio and problem solved. Just gather an ESC, receiver, and radio , then take out all the other stuff (maybe even sell it) and you're done!
BTW: when I mention losses, I'm not really concerned too much about heat (assuming the stock controller does use PWM to the motor), but more about system efficiency (reflected in runtime) and adding points of failure.
Also, what are you going to use as an LVC? The converter masks the true state of the cells until they just all-out stop. The lack of punch and/or speed of a direct battery to motor setup would be an indication of charge state, but you'll not see it with the converter since it outputs a steady 13.8v.
These converters must simply be high current buck regulators, essentially "large BECs", and are likely to fail the same as any BEC; when it does, it outputs full input voltage (18-36v in your case). Just something else to think about.