I've been looking into lowering springs as a possible alternative to going directly to KW V3.
I've found that the spring rate of the Eibach Pro-kit is as follows:
Front: Progressive spring, initial rate is 23N/mm and the end rate is 34N/mm
Rear: Progressive spring, initial rate is 70N/mm with the end rate of 140N/mm
34N/mm is only 194lbs/in and the Eibachs claim to be 15% stiffer than OEM! No wonder the Z4MC squats and pitches so much. I won't be fitting Eibach springs.
A coilover kit for the Z4MC would normally use a spring rate of around 400lbs/in at the front and that would considerably improve the handling.