MFI is classic for this mate, injection with the drawbacks of a carbeurettor. How did you increase the fuel pressure???? Have you replaced the regulator for an adjutable one?

Two mods which are done to help with high end fuelling, are putting a vacuum hose to the standard pressure regulator (?? not too sure if thats an option for you if you have changed it), and theres a screw on the back of the hydro electric pressure actuator which can be adjusted (the hydro electric pressure actuator being the sole component that controls enrichment of the mixture). I dont know the exact procedure and i wouldn't recommend having a play with it blind so to speak.

Also, check your Throttle Position Switch is fully working, if that breaks the car will never give full enrichment on boost (the switch being the sole component that triggers full enrichment)