One thing i've been meaning to ask for a while, when you guys upgrade the rear suspension to coilovers, what do you do about the existing rear spring? Do you change the OEM one to a different quality one?
Unless it's a struts only kit, a coilover kit will come with different rear springs. An example of installation pictures of a KW V3 sustem on a Z4M -> http://www.billswebspace.com/BMWZ4MKWV3SuspensionInstallationDIY.htm