so I was using the wheel/tire size calculator over on e90post.com because for whatever reason there isn't one here yet. but here's some interesting information
stock base wheels: 17x7.5 et37
ZSL/ZML/ZLL wheels: 18x8 et34
ZSL/ZML/ZLL 19" or all ZMM staggered set ups: 18x8 et34 or 19x8 et36 front and 18/19x8.5 et47 rear
Going from the standard 17s to the 18s pushes out the wheels at all four corners by 9mm
Going from the standard 17s to the 18 staggered set up pushes out the front wheels by 9mm but the REAR BY ONLY 3mm!!!
Going to the 19 staggered set up pushes out the front by 7mm and the REAR BY ONLY 3mm!!!
interesting stuff. given this I am really starting to think that a 235/50 17 or 235/45 18 square setup is best. Maybe 245 but the tire sizes will be a bit shorter. Yea the 235 is taller, but it should fill up the wheel wells better.