Contact Kale Bushmeyer at TC Kline Racing. He's successfully campaigned a Z4 on the national Solo Tour and at SCCA Nationals. I was looking at having them build a set of dampers for me, but have held off while I tune the driver a bit more. Here's what he had proposed for me. This was late 2014 so the pricing may be a bit higher currently.
kale@tcklineracing.com
My query to TC Kline:
I believe per the 2014 rule book, struts with adjustable spring perches can be used but they have to be used with the stock coil spring and the lower spring perch has to be at the same height from the spindle and the OEM strut.
Kale's response:
Here are the options that you would basically have:
You can purchase a Sport shock with the OEM spring perch and we would gut the internals and turn it into a DA shock (this is just for the front). It would be about $300/side to modify the shocks, plus the cost of the sport shock $224.50. So it would be approximately $525 per side for the fronts. The rears, you can just bolt up with the stock springs in their normal location.
Your other option would be to purchase a pair of our Double Adjustable Struts for the front, and then you would need to have a machine shop attach your stock spring to an adjustable spring land that can be threaded onto the threaded body of the Coilover. The advantage to that would be you could eventually take off the stock spring and it would be ready for an adjustable spring land with linear spring rates. The Double Adjustable Coilovers would cost $539 per side, plus whatever cost to attach the spring land to your stock springs.