One of the guys Marius that used to be on the forum had 19x 10 with ET 24 on the rears. The car was originally lowered 30 - 40 mm but he eventually went to coil overs.
Loads of Pics here. He stretched some 235/35/19 rubber on
http://www.z4-forum.com/forum/viewtopic.php?f=6&t=41561&start=120 page 9
http://www.bimmerforums.co.uk/forum/f223/my-z4-build-t125559/
http://www.alfa.lt/straipsnis/15147526/premjera-kazinak-bmw-z4