I've done a little mucking around with pressure gauges, temp sensors and the like.

More boost does increase the efficiency of your engine (higher effective comp ratio) which results in more power for the same fuel. But the back pressure in your exhaust manifold also starts to choke the engine more (my tests show a rough 2:1 back pressure to boost, gets worse near the limit of your turbo).

So basically you reach a point where you start to go backwards. I'm running 15psi without much intercooling (10%), the old turbo would blow up to 20 but it was an exercise in futility.