Circling back on this, looks like 19s are the way to go for a staggered setup. The 285/30/18 and 295/30/18s are just too short compared to stock and front 265 or 275/35/18s.
19s with a 9.5" front and 10.5" rear will allow the tires and sizes below:
R888R- 265/30/19 & 295/30/19 or 305/30/19
RE-71R- 265/35/19 & 305/30/19
R7- 265/35/19 & 295/30/19
Offset on the 10.5" will be key to fit on a lowered car. Supreme Power fit 295/30/19s in the back on 19x10.5 ET45's on a slightly lowered car here:
http://f87.bimmerpost.com/forums/sho....php?t=1455216
I'm going to try some test fits this week and will report back.