The controller sets the amperage, limits it, so a controller upgrade might get you 100-200 watts. It's risky (hard on the motor) and 200 watts is not that much. The odds are the motor already runs around 750 watts, anyway, just because the ratings are worthless. A big company like Pedego is not going to risk a motor and controller way over 750 watts, and 1200 would be a noticeable bump.
The way to go is to build, since most builds are 1000 watts and up. Why mess up a nice bike, and that's a premium motor with a well integrated torque sensor PAS. Most people want more speed, but the regulations are not set up for it. If you feel you want 1500 watts, you are saying you want some kind of light motorcycle. I hope this will start to develop. I would be happy to keep my ebikes and have a 5,000 watt light motorcycle.