Ok so if you put a little 177 roots on a 502 and let's say it makes 8psi of boost spinning like crazy (creating heat) and you put a 10-71 on that same engine and turn it only fast enough to make the same 8psi of boost, do you really think the power output would be the same? There's a lot more to it than boost. Hot air takes up more space, but makes less power. I'll take 12psi of cool boost over 16psi of hot boost every time.
The HP per psi of boost isn't going to work for every engine, way too many variables (IMO). Another example same engine as above with a 10-71 making 12psi of boost not put a cetrif on it blowing through an intercooler make the same 12psi of boost (since the roots won't be working that hard the losses from driving it should be minimal) which one is going to make more power? To make it fair put a centrif that is a little too small so it has to work harder, still going to make more power per PSI based on inlet temps.
Interesting question I just don't see how you can havea formula that will take into account all the variables.
I think alot depends on when your heads start to become restrictive. At one point in my dyno session one pound of boost was worth 60 hp and as the boost increased at the end of the pull one pound of boost was only worth 8 hp . Once the torque peaks and starts to fall is an indicator that your horsepower is going to start falling per pound of boost from what I discovered. The question to me is if you start to spin the blower faster and change the torque curve how that will affect the hp if at all ?