Sometimes looking at Dragtimes database can help. Not much for stock Z4's (the ones denoted with # on them), but it appears that the 06+ si is slightly quicker and faster in the 1/4 mile than the earlier Z4's. Not enough data, but there is probably a small gap. It does not seem enough to really worry about if you find a car that has the colors and equipment you want.
http://www.dragtimes.com/BMW--Z4-Drag-Racing.html
Jeff