A general FI question

Without knowing the exhaust, intake, heads, etc... ANYTHING about a car.

Just turbo and PSI. It's damn near impossible to get an idea of what kind of power he's putting down, no?

Because if he has a extremely worked over motor and all the supporting goodies... wouldn't his PSI be lower in general?

You are correct in saying without specific info there's no way to get an accurate guess on the HP of the car. Take for instance a stock LT1 and put a P1SC with 6#'s of boost, that same P1SC running 6#'s of boost on a highly modded motor would obviously put out more. Basically stated, if you want a ballpark guess on RWHP, you'll need all the specifics of the cars setup. Even then it will only be a ballpark guess.

As far as the psi thing goes, boost is the result of restriction. Having that said, too much restriction won't necissarily equal out to the most possible boost level. It's getting just the right amount of restriction to your application for the desired boost level. Most of the restriction falls on your cars heads.