I've been thinking a lot about this backpressure issue. If you look at Kjewer1 setup, he is running a 96mm turbine wheel to drive his 75mm compressor. On the BMW setup you are running an engine 1.5X the displacement with a 88mm compressor being driven by a 92mm turbine wheel. It would be my best assumption you need a larger. Turbine wheel on the large turbo to get the backpressure numbers in check.