Yeah as above 170-190bhp on a stock T3 is normal, the boost is dropping off presumably as the revs increase yes?
This is beacause as the RPM goes up the amount of air flow needed from your T3 to hold any given boost pressure increases, and as it is a stock T3 on an engine with a ported, cam'd head, it cant do 17psi.
What is it holding, 11/12psi?

A upgraded T3 is your answer