There is a security valve in the intake manifold that pops open when you exceed 15psi. This is not your wastegate sounding like a BOV, it's the valve in the manifold releasing the pressure out into into the engine bay. You can remove it and replace it with a plug and then you may get something like 18-20psi but don't expect more. But I wouldn't do it since there is a reason for it being there.

1987 SWB Pajero 2.5 TDI on 31" rubber waiting for a hip replacement and bigger shoes