Number of air molecules is THE determining factor. When you change pressure and nothing else, you can assume the amount of air went up so in that case boost equates to power (albeit with efficiency losses).
But from engine to engine or turbo to turbo, intake pressure alone is meaningless, and like Zellamay said your engine won't "benefit" from increased pressure.
(Think about trying to compare a 20psi Evo to a 12psi Z32--those numbers don't mean a thing. Same with different turbos, or the same turbos in different condition--the fact that you measure the same psi just doesn't mean anything.)