Interesting thing I learned today that I haven't seen or noticed brought up before. The steering rack is different between the coupe and roadster.
Z4M Coupe 12.8:1
Z4M Roadster 13.7:1
http://www.z4-forum.com/forum/viewtopic.php?t=67644
http://www.m3forum.net/m3forum/archi.../t-300883.html
http://forums.m3cutters.co.uk/showthread.php?t=108448