What Sean (xr2wishy) says is spot on!
On my old engine that was a very similair spec to yours my stock T3 would peek at 17psi and then gradually drop to 11psi as the revs increased, you could force it to hold a bit more with an electronic boost controller, but really a higher flowing turbo is what is needed.
If this is your problem of course!?

IMO very rarely do we see weak valve springs on these engines, as they are really heavy to start with!