A short update
The turbinish whine must have been the turbos running higher boost and getting hot due to spirited driving. It seems the noise is less when lowering the boost by switching from ESS stage 2 (390bhp) to stage 1 (350bhp).
Little known facts about BMW: 1) The N54 is still in production and used in the current Z4 35i and 35is. 2) In a Z4 you can get sport+ steering combined with normal suspension etc by selecting DSC=off. (Update: DSC=off also offers sports steering in the M135i) 3) All Z4s with 19" wheels are still only offered with the worst performing OEM tires ever sold with a DCT equipped BMW (Potenza RE 050 RFT).