As an experiment I actually just bought a set of $330 CX racing coil overs for the Z4 and installed them 2 days ago. By comparison they have:
- Coil perch and strut body adjustment
- 4 coil rear springs to prevent dead coils
- Included front strut mounts (non-adjustable at this price range)
- Much more comfortable ride that stays within the shock optimal operating range regardless of ride height
I'm actually very impressed with them. Would highly recommend to anyone looking. Who knows how well they'll hold up in the long run, but right now they're a great solution.