I have a classic with 2.5L, uprated internals and modified heads and VF22 turbo. The pressure builds very nice and fast and it peaks at 4200rpm, 1.4bar, after that it starts to drop and ends up to 0.95bar at the 7500 redline. I tried everything, I changed the wastegate to a JUN one and checked all the connections, the turbo is ok, only 2k miles on it.
Is there a chance that the turbo is too small for this engine and max out at the high revs?
I saw some threats in the NASIOC, and there are people that use these turbos on 2.5L with not so much pressure drop.

 

Even if you hold more pressure you won't make any/much more flow as the exhaust manifold pressure will rise.

Even my TD05-20G with some octane booster made 434 BHP at 1.27 bar at the top end on my 2.5, but there was no point trying to hold more boost. The GT30-11/P20 hybrid I tried would hold 1.4 bar to the top. Only the rotated GT30R-12 was truly happy holding 1.5 bar+ to the top.