I am having exactly the same problems with an Internal gated 2835 ProS. Have fitted a 22psi spring with no real help. A new theory is that the head can not flow enough air to maintain the higher boost (turbo efficiency). I am running 20psi with the same power result however drops back to 15psi at high revs. I believe Cams (more duration) which I am in the process of getting or possibly even just cam gears may give us the flow we need to hold boost. The bigger turbos like the 3040 overcomes this by providing more pressure at a lower turbine flow rate with more lag.
Have you got cams I didnt look. However this all theories so let me know if you come up with any fixes.