I tried a sidewinder in my CRT.5 with a 5700 CC motor. It ran great on 2s, but terrible on 3s. Then I read on the CC site that you're not supposed to run a 5700 on 3s with a sidewinder. Oops.
A BEC might help it run that motor on 3s, but I just swapped it with an MM out of my Axial crawler and now it works phenomenally.