our work is 70% BMW tuning now. We have stage 1,2,3 and 4 available for the N54 and stg 1,2,3 for the N55.
Stage 1 is a custom map makes 370-380bhp and 380 ftlb (makes 12 psi)
Stage 2 is a custom map + decat downpipes (still passes MOT) and will see 400-420bhp and 430-460 ftlb depending on the boost level you want. (13-15psi)
Stage 3 is a stage 2 but with meth injection, intake, more mapping and will see 460bhp and 530ftlb. (makes 16-17 ps)
stage 4 N54 uses a hybrid turbo with all the bits from stage 3. We are waiting for more takers for development work but early results are 500+bhp and 580 ftlb Big power N55's are showing signs of reliability issues, the N54 is built to a massively high spec for a road engine, being just a dry sump short of the spec of a lot of race engines.
best advice I can give is to go Auto/DCT if you are going big power due to the torque figures. Passenger rides in our stage 3 (soon stage4) 135i are available to serious customers.