Yep you have the classic 'S' in the power curve at the top end, the load scale on the R34's drops timing by around 10 degrees as you go over a critical airflow point ~12psi on a stock GTT will see this happen. Funnily enough you have got almost the same power my car had with exactly the same problem
As Adam said, try dropping the boost a tad on the dyno, you should see a nice curve form when you get the boost right.
The only way to go from here is to get an aftermarket computer so you can eliminate the factory retard at higher boost, on a positive note it stops you trying to squeeze too much out of the stock turbo