actually we have seen this problem on the dyno a few times , had a rb25det with front mount and good flowing exhaust suffer from this problem , even removed the actuator and opened the flapper valve all the way and loaded the car up on the dyno still made 13 psi of boost and tapered off to 12 at 6000 rpm , explain how and different actuator will help when the gate opening(hole) is physically not big enough to get rid of gases (also this vehicle made 200 rwkw on 12psi with stock turbo)
regards,steve