Mine had a built engine, head work, cams, cam gears, 4" exh from front pipe back.
I have seen same setups making 290, friend of mine in his R32 GTR is making 328 with more boost, was tuned from a place in NSW just before he bought it, so i would say a little more boost and head work makes the difference.
Simon from NT Performance Turbocharging in Footscray, they had built some pretty serious GTR's and GTS-T's thats why i took mine there, if your keen i can pass their number on and you can have a chat to them about what was done.
I dare say that yours with head work perhaps a slightly bigger exhaust would make more