The supercharger applications I have seen have a boost knob or lever that controls a by-pass valve, I guess.  So, yeah, the supercharger's output is dependent on pulley sizes, etc..But they are also equipped with some kinda boost valve to keep the motor from "over-boosting" itself into oblivion, which is what all the Porsche 944 turbo guys used to do when they tried to beat my stroker motored 928...They would watch me go by, then twist their little knob for more boost..till the headgaskets blew...
  I think on a SC motor, your pulley ratios determine the max boost you can get, but you can manually control the amount of that boost that is sent into the motor. If you need minimum power, you waste a bit of mechanical energy by pumping air out of the boost gate..If you want the power, you mix in some fuel and pump the whole mix through the motor....Also, on a SC motor, there is said to be no lag...power is directly related to RPM, not dependent on exhaust to spin up the turbine to power the pump...it comes off a belt..
  Don Hanson