I've found out that although the powabyke is rated 200W it is continuous rating, its peak output is actually 700W. Also as it is a brushed motor it gives better torque at lower revs than a brushless one. And the hub has internal gears which keeps the maximum speed to around the legal maximum of 15mph but also helping with the hill climbing. It may be that it is the best choice after all.
Do you know what the peak power is of your 500W motor (and make)?
Does anyone have figures for peak and continuous power for any of the 500W and 750W motors?