Just reporting in what I've found so far.
The
750W GM on a 26" front wheel and a 36V LiIon battery can get me (100kg with gear) up to 42km/h with light pedalling (I mean not enough to get sweaty here in Singapore where it's 30 degrees Celcius, during my 5 minute flat-and-straight run). Without pedalling, ~30km/h on the flat.
This would be good for getting to work and back, if it was legal to ride.
However I still have to pedal a little on the hills. I don't know the gradients but 99% of people get out of breath just walking up them. If you need to judge my target hill, you can see it on Google street view:
http://maps.google.com.sg/maps?f=q&source=s_q&hl=en&q=Nanyang+Avenue&sll=1.347053,103.685285&sspn=0.006071,0.011362&ie=UTF8&t=h&split=1&rq=1&ev=p&radius=0.47&hq=Nanyang+Avenue&hnear=&ll=1.349584,103.684984&spn=0.006071,0.011362&z=17&layer=c&cbll=1.349138,103.684395&panoid=f49AXQdSj1MP9JE7iFKz3Q&cbp=12,219.21,,0,5The
250W GM mini-motor on a 20" front wheel and a 24V lead acid battery (2x 12v 2.3Ah units, just for testing) is really, really slow. I haven't measured the speed, but I estimate no more than 20km/h with light pedalling. Without pedalling I'm lucky to get to 15km/h on the flat. It feels slow, but is enough to balance reasonably, and ride well.
However it's quite grunty on the hill. In fact, it seems just as good on the hill as the 750W motor.
Another advantage, it's street legal (or will be within a few months).
However I really don't like the fact that the minimotor wheel is completely off alignment. It's far from symmetrical, jutting out about 1" on one side. Even with lots of washers on the other side, and pulling the forks apart, the brakes still aren't coping well.
Much better to have a disc brake - but I don't have the fittings on my fork and I'm told it's not possible to retrofit disc brakes if the frame fittings aren't there.