Speaking of the MT1s, I've been looking at them as well. Not sure if I'd go with Tirerack's recommended square setup (18x8.5 et44), or buy 4 in the 18x9 et35 (doesn't show up for Z4 but will show up for the e90 328i). They get great feedback... too bad they're not as cheap as the 18" TRM C2 (only $162/wheel) but not as light (20.8lbs vs 18.5lbs).
http://forum.e46fanatics.com/showthread.php?t=560272