We've been watching Z4's too. Roadsters and Coupes. Not the M's. (We still have aftermarket rims from our 2003 M3 Dinan 'vert. Wish I could find a car to put them on.
)
They didn't fit on our 335 'vert. (darn car was about as fast as our M3!) And both were faster than our 850i!
Love the lines of the Z4's Wish the M3 engines were better.