Today I swapped the front and rear wheels and tires so that I could test the fitment of the 18x8.5 ET37 stock Style 313 wheels and the stock Michelin PS2 ZP 255/35ZR18 tires on the front.
Then I drove down to a local school with a vacant parking lot and did a few laps, concentrating on some tight turns, and a few hard stops. I didn't feel any rubbing or bumping. The only time I heard any rubbing was when I had the wheel turned all the way to the left and backed up.
When I got home I turned the wheel full lock both right and left and discovered that on both sides with the front of the tire turned inward that I couldn't fit my index finger between the tire and the front of the fender well, right in front of the front side radiator grilles.
The Apex 18x8.5 wheels have a ET of 38 so compared to the stock rear wheels and tires mounted on the front the Apex wheel will move in 1mm with 1mm less clearance to the strut and 1mm more clearance from the fender lip.
18x8.5 seems to be a safe size, I'm wondering about the 18x9 ET42 which would put the wheel 11mm closer to the strut and 1mm closer to the fender lip.
Mine: 2006 Z4 Roadster | Interlagos Blue Metallic | Black Extended Nappa Leather | Carbon Leather Trim | Premium | Navigation | Heated Seats | iPod Adapter | Purchased 7/19/12
Hers: 2011 335is Coupe | Le Mans Blue Metallic | Black Dakota Leather | Dark Glacier Aluminum | 6MT | Premium | Convenience | Heated Seats | Navigation | Harmon Kardon | iPod/USB | European Delivery 5/12/11