I will try,
Power is measured in Watts=voltage times current, so from basic math if you increase voltage you use less current for the same power, right?
Power is a rate at which electrical energy is converted into mechanical energy inside your motor.
Power is a consumption of el. energy, the more rated power of the motor the more el. energy is going to be consumed, so 800W motor is going to consume twice the energy than 400 motor.
Performance will improve on 800W motor versus 400W one but energy from battery will be depleted twice that fast. So yes faster spped but for shorther time.
Amphour /Ah/ has nothing to do with speed, it is capacity of battery= how long before battery is deplated.
From 20Ah battery you can draw in theory 20Amps for 1 hour or 1Amp for 20 hours or 5Amps for 4 hours, etc.=battery capacity.

miro13car