As mentioned above:
For Turn-in: Add a bit of Camber, Slightly stiffer rear sway, and the appropriate choice of Springs & Struts/Shocks or go to decent coil-overs:
For Continuing the rotation after turn-in, put a LSD (same as for a 335i) in. This will keep you from losing power in the rear, therefore note getting a higher ration of power up front. That is a big source of the understeer with these cars. I have a Wavetrac, and the rear-end really hooks up well now on the track.
2009 335i xDrive 6MT LeMans Blue
Dinan Intercooler, Dinan Oil Cooler, Dinan Cold Air Intake, Dinan Stage3 SW, Dinan Stage 3 Suspension, Dinan Front Strut Brace, Dinan Exhaust, WaveTrac LSD (rear), D-Force EmPowers, PFC BBKs, UUC SSK, Macht Schnell TMPS Override