Air compressor Question Again..........

I'd take this parameter as gospel: don't exceed about 1000 rpm. Any combination of speed and pressure that you can get out of a motor without exceeding that rpm should be fine (i.e., you can max out the pressure at max amp draw at a slower rpm, or you can max out the air delivery at a faster rpm at maximum amp draw). Personally, I'd shoot for a cut-out of 130 psi @ max amp draw. That should give you a decent compromise between pressure and delivery.

If the 1-1/2 HP you've now is not overheating or popping the branch circuit breaker, the motor is NOT lacking for power and being dragged down too slow as a result.

You can verify by taking a current reading on one leg the power feed to the motor. It should be very close to the "full load amps" or "FLA" marked on the motor nameplate just before the pressure switch cuts the motor off.

If you want more air or faster pump-up cycles, you'll need to spin the compressor faster. And that takes a larger HP motor fitted with a suitably larger diameter motor sheave.

For what it's worth I've got an ancient Kellogg 2-stage compressor with a 2 HP motor on an 80 gallon tank. It takes it a good 8 - 10 minutes to fill to 150 psi from an empty start.