on the pro roller, it looks like you have the diff turned around. you could use a normal sized pinion if you turned it around. i took some pics for you:
if you use a steel spur it'll work great, you just can't gear it over a 45T. so is that a big bore pinion in the pro roller too?
i think i'm gonna try the same build you did for running 8S with the HV-110. that 2 speed gear looks like a great idea. also, is there a way to get the motor closer to the spur gear? i was over at impakt rc, and i couldn't find any variations for a motor mount. would it just mean re-drilling the holes in the chassis?