The 800w hub-motor is probably the Q128H. It's a freewheel motor for freewheel gears. The Q128C is the cassette version, but only rated at 500w because it's a bit narrower for the cassette. Whichever motor you use, you get the same power with the same controller. 250w, 500w, 800w are only ratings, i.e. it's the controller that decides the power not the motor, but obviously the motor needs to be tough enough to cope with whatever power you give it. At 48v, Q128C is happy with 22 amps. the Q128H can do 25 amps.
22 amps at 48v is a lot of power. i think most riders would be happy with that.