Boost on a supercharger is nothing like boost when turbo charging. Compressed air ( boost) building in the manifold is basically air the engine can't ingest. The more boost the more the blower has to work against it to make power. ie. spin the blower as fast as possible and make the motor as efficiant as you can , I have found most power to made around 8.5 psi while not dealing in too much overlap. Dropping boost at higher rpms is normal with a eaton blower.I would like to see what the car is doing on a dyno at that psi. Also what is the max rpms of the blower?