OP - You haven't commented on the Dinan Spring recommendation that a few have made. I plan to keep my ZCP suspension on the M3 (although it is a little floaty for my tastes at 135+ MPH on the track). Having said that, I upgraded my 335i with Dinan Shocks and Springs and it totally transformed the car. Dinan has suspension tuning down pat and I think a non-professional or at least VERY experienced enthusiest would have trouble tuning a coil over or sleeve over with adjustable shocks as well as Dinan's products come shipped from them.
I'm sure the shocks had a lot to do with the improvement I experienced in the 335i, but just getting their springs seems like a no brainer. If you wait for a sale, the cost isn't really that bad either. Sometimes they have 25% off which neutralizes the normal Dinan premium.
Also, never underestimate the importance of suspension travel. On the 335, Dinan incuded update mounts which kept stock (or near stock) amount of travel.
Whichever you choose, good luck, but I expect you'd have ZERO regrets if you choose Dinan. Check how much they think it will lower. My 335i was only a tenth of an inch or so lower.
M in love