I believe that in general bmw frowns upon turbo and super charging. thats why they typically dont put them in their automobiles.
"BMW considers turbos or superchargers to be low-tech shortcuts to power, horsepower solutions unfit for a flagship engine. Blowers, they say, add weight and complexity and reduce throttle response. Ask a BMW engineer if the V-12 might one day be turbocharged, and you'll get a scowl and a curt, "Nein!""
Quote from Car and Driver @
http://www.caranddriver.com/article....rticle_id=4183
of course you cant believe everything car and driver says, but i think it is mostly right in this case. and also since your car is an automatic i think it would be almost pointless, even though you have the steptronic. . .