I don't have any first hand knowledge of HPC, other than what I've read and watched on YouTube. I believe there is a HPC sub forum here. As to whether the bikes are legal, you should be able to find the information you need here: https://en.m.wikipedia.org/wiki/Electric_bicycle_laws
It's an amazing setup, I'd have loved something like this in my motocross youth. No gas, no oil, just ride and plug it in at the end of the day. A 60 mph, 8 horsepower (6000 watts) bike would likely be confined to private property and places designated for off-road motor vebicles. It would be a blast to ride though!
I looked at the laws it says 20 mph and 750w max power. I called hpc and they said they put a switch on it to reduce the speed and the wattage I can change when I want. Also it's under the 100 lb limit. I asked about making it pedal assist he said they can do that but would have to remove the Schlumph drive.? Not familiar with it. He said it is what will allow me to pedal at 60 mph. In your opinion would that make it legal?
I suspect a cadence sensor cannot be added to the Schlumpf bottom bracket.
As for power, adding a user interface to turn the speed or power on, off or down doesn't legalize the bike to be ridden on bikeways. An ebike is either class 1, 2 or 3 and manufactures are required to permanently label the bike as to it's class if it's to be sold in California and a few other states. If it were as easy as installing a switch, we'd see a lot of electric motorcycles on our bike paths.