Has anyone upgraded from the Z4 Coupe E86(2006-9) 325mm x 25mm, to the larger E46 CSL front discs, 345mm x 28mm thickness and the same 52.5mm overall height, when lying flat? Obviously, calipers need changing as well...
Manic, I replied to your question on the "M" thread in case you didn't see it!
CSL front discs have M Tax so aren't cheap! I'd be inclined to keep what you have and upgrade to 4 pot calipers as the M ones are still only single piston.