[TurboKraft]

If I'm understanding correctly from your datalogs:
- 0,4 Feder => 0,36bar @ 2847UPM; 0,49bar @ 4227, 0,98 @ 6018
- 0,8 Feder => 1,1bar and then shut off for safety

Since changing the wastegate spring isn't resulting in lower peak manifold pressure, I recommend:
1. check that the wastegate's control lines are connected correctly -- always review the basics first...
2. get rid of the "wastegate muffler" and repeating the test. Those pipes (resonator / spark arrestor) severely limit exhaust flow out of the wastegate, and we have had them produce significant boost creep. This is also an easy test.

After that, I would normally suggest inspecting the wastegate to see if the valve travel is being limited in some way (obstruction?), or if the diaphragm has a leak (pressure check), but since you just had it apart, I will assume you checked both of these items.


Headers:

The GHL headers have some inherent boost creep because of the unfavorable exhaust flow from the turbo collector into the wastegate pipe. It's a design that is simple to construct and thus low cost.

B&B headers do, too, but inconsistently -- some better and some worse -- because the angle of the wastegate inlet pipe off of the turbo collector is inconsistent. I remember on (EFI) engine that went right past the 11.6psi (0,8bar) spring and made 16psi manifold pressure. It wasn't a problem as that engine was built to run 18psi daily on pump gas, but it was incapable of running *less* maximum boost.

Fabspeed headers also have some boost creep because of the same basic design.