well i tried to get some input directly from FK, and ended up getting an email from a rep via ECS tuning.
i inquired about spring rates and details, and all he was able to respond was the following...
Hello,
The only information we have on these are directly from FK, they are made to fit your vehicle. Hope this helps.Thank you.
ehh, i expected more from ECS tuning then that response..
anyhow, this is the link to the specific product they identified for my 06' Z4MC:
http://www.ecstuning.com/ES2142892/
iono, but it seems too much of a risk to take for $1349~...