Well from what I spotted in 2 minutes on google it seems you could put a 6100Kv motor on it as you suggest, so I'd stick to that limit. It's not a very high spec esc in terms of racing, but plenty good enough for getting a car running and bashing around, maybe some club racing even, so not worth the risk of blowing it!
It is a sensorless only ESC though so you can't use a sensored motor with it.
As for motor size, what are you using it for? Racing or bashing? Big space, little space?
If your racing then bigger/faster isn't always better, much better to get something suited to the space your running in...but if your bashing and just looking for high speed in open spaces then go to the 6100 and enjoy...just try not to hit anything...the B4.1 will easily handle it, but a fast car is only fun when your car's not broken and the faster it's going the more damage you can do
