The thing I don't get Ian is this. You're after 'more torque' which means a curve which has more area. Now the GTR engine/2.6L lacks grunt south of 4000 - 4500 RPM, Yet you're using a turbo which is proven over and over again that simply does not work in that area.
In my own experience I have picked up 100 RWKW at 4500 RPM. That means more torque yes?
After being there and used them, I couldnt care what capacity I have behind them I wouldn't use them.
You say you want more power than the -5's can produce... What is their upper limit? I was at 425RWKW with cam timing with hurt its top end. A simple twist of the intake cam would reel off 450. Thats at 24 peak tapering down to 22 psi topend. (although its on E85, however its a super safe tune)
For you to make 303 at 18 psi on RS's there must be a serious serious restriction somewhere. It aint the airbox. Exhaust? Possibly, but wouldn't be the full cause of it.