I'm running a gt3076 on a rb25 manifold with a 38mm external gate welded off it. It does not spike at all but in the top end it drops from 19psi to 16,15 psi.
I'm using a blitz sbs dual solenoid ebc.
Might need to test for a boost leak !
But if any other ideas come to mind on what it could please let me know !