your saying you only get 2-3 psi at full out full throttle?
sounds like a boost leak somewhere. Where you picking up you boost sensor?
pre-turbo exhaust leak is possible, easy to check, im sure you can see soot back there if you look up under, it would have to be a pretty major leak.
i would check your hose connections at the intakes below the cross pipe mine would always blow off until i rigged up something new.