At least your expectations don't sound unreasonable.
Tire size doesn't matter when using a friction drive. The torque and top speed is dependant only on the diameter of the drive roller.
I've got a 25cc Homelite motor on my 1973 Schwinn Continental 10-speed. It's got skinny 27-inch, smooth tires, and rides in a "racing" position, so that helps bring my top speed a little over 20 mph. Drive roller is 1 1/8" diameter. But the old bike is heavy (and I'm about 210 lbs), so a lighter person on a modern mnt bike might do just as well.
I have to pedal when fighting the wind or going up hills; I don't mind. I get great mileage (and need the exercise).
A 1" roller would probably give you good results.
If you have perfect efficiency (no loss due to slippage), a 7/8" roller at 6000 rpm will give you 15 mph. These little motors can definitely handle 6 grand, but they have little torque and might top out below 6000 even on level ground with no wind.