This is exactly what I'm looking into.
Yes, to the others, we're talking about km/h. (Voltbike is a Canadian bike). The bike current does 40.6km/h in it's current state so something like 50km/h should be achievable since it's only 10km/h more.
The current rear hub motor should be able to do 1500 watt bursts (it doesn't seem to get warm even when doing sustained 40km/h for long periods of time so I'm fairly sure it's well within limits).
The controller, as you stated, handles up to 25 amps peak AND the big issue (or rather, challenge) is that the amp doesn't change EVEN when you change it from the display. It seems locked at 15amp regardless.
I imagine that if we could get adjust the controller from 15amps to 18-20amps, then we could achieve the desired results. I'm not sure if the 850C display would be any better at changing the amperage since it seems locked at the controller.
Is there a way to unlock it at the controller level? (Can we even program the controller individually? Is that why some people are using a USB wire/adapter?)
I also saw that someone had a phase regulator installed on an older Voltbike. Is this what it was for?