I've used 24volt 600watt motors to drive a large electric loco. Actually used 4 of them, one on each axle. The motors were originally from electric scooters and probably made in China. Very nicely put together, even if some of the engineering was a bit crappy.
As part of the project I conducted an efficiency test of electrical power in vs mechanical power out test by having one motor drive another, which acted as a generator ie an elctrical brake rather than a mechanical one. By applying resistive load to the generator I could measure it's electical output power. At the same time I measured the input current and voltage on the drive motor to measure its electrical input power. Efficiency transfer of motor input electrical power into mechanical power and then generator efficiency of mechanical power into electrical power could thus be determined by halving the motor to generator electrical transfer ratio.
I no longer have the actual data but I can confirm that the generator delivered 600 watts of electrical power. It therefore follows that the drive motor was producing in excess of 600 watts of mechanical power.
I used a 4QD controller (rated at 36 volts/300amps) to drive the motor. They are a UK based company but will export overseas at very competative prices. Their web site also contains loads of good information on typical applications 4qd.co.uk
They also have USA based distributors
jack