I'll have to get in touch with them to see what they say about it, they offer no BSP for registration in the dropdown box.
On a sidenote, here's a quick comparison I made today...
First up, front strut from the Z4 after removal
Second, rear shock from the Z4
Third, a random old bilstein strut insert from my e30. If memory serves it has around 70,000km of usage (with multiple trackdays in the mix)
Call me crazy, but that front couldn't have been helping my understeer issues
