Well, I tried to find my aerodynamics and thermodynamics of jet and rocket engines textbook last night but couldn't find it.

So, I couldn't find an answer to questions that I was looking for. (Don't ask me how long ago I had this text. Let's just say it was long enough for me to forget.)

What I was looking for, and still haven't found the answer yet, is exactly how the a centrifugal compressor works. What I mean is, is the boost, say 5 psi all the time, or a relative 5 psi. Let me see if I can explain.

If at sea level you add 5 psi to the ambient pressure, then when the ambient pressure is 20% less at higher altitude do you get 5 psi over the 20% less, or do you get 80% of 5 psi, i.e. 4 psi of boost? I strongly suspect it is the latter. In any case, for a centrifugal compressor, you don't get full boost until it is moving fast, i.e. in the top RPM range of the engine for a supercharger.

Unfortunately, the boost is not a simple linear function of the rpm of the compressor. It is an exponential relationship, probably with the exponent being a ratio of the heat capacities of air (1.4). But, at this point, I just don't know. So, with all of that said ...

Increasing the speed of the compressor with a smaller pulley will get you more boost (up to a point). How much smaller do you need to compensate for the increased altitude I cannot say. It is not a matter of, ok, I lost 20% density from high altitude so I need to spin this sucker 20% faster. I will keep researching this.