Kit - I think the problem with your sums is that you have assumed "constant power" with speed change. I doubt that this is true. Like internal combustion engines, torque tends to rise with speed and as power is the product of the two, max power occurs somewhere towards top RPM/top torque. Electric motors are different, especially with the fancy sensorless vector controls and what have you (and I really don't understand exactly how that works!) but I suspect that the truth is closer to "constant torque" across the usable speed range and then falls. Certainly mine drops off sharply below around 7K RPM, based on observation (i.e. not accurate measurement).