the boost control itself seems to be a bit dip-py aswell, floating around 10psi..
ive got 3" split dump, 3.5" exhaust, pod filter, ARC 450 FMIC, Microtech LT-12 running a pretty stock config, except its a bit flat at the start of the rev range, and i get 270RWHP
(i dont have a picture of my dyno graphy handy, but its pretty much the same as yours, except at the start it gets more power pretty quick, then midrange it flattens off, then it jumps heaps towards the end, and begins to curve down a tad near redline, peaking at 270RWHP at about 135km/h, altho it was dyno'd in 3rd gear)
so it should be safe to say you could get another ~30hp, and a more steady graph with some decent management.