[v8volvo]

Good to fix the leak but it shouldn't cause boost to fall with higher RPM. Something about the way this setup breathes isn't right.

Are you able to check the boost pressure ahead of the IC? Eg from the wastegate signal line? You are sure the wastegate is seeing the same pressure signals the intake is? If there is restriction in the IC then that's going to cause the exact issue you are experiencing. Boost controller and wastegate the right parts installed the right way, correct spring choice, etc?

The 3" inlet, is that direct from atmosphere or is the snorkel still in place?

How recent is the fuel filter? If there is fuel system restriction causing retarded timing at high RPM, that can cause issues. Pinzgauers have a primary fuel filter under the body in addition to the main secondary filter on the engine, are both of them fresh? Tank pickup clean, tank vent OK, etc? Checked fuel inlet vacuum under load?

How does it start and run otherwise, basics like cam timing and valve clearance are all right on?