The difference between the 15" and 17" wheels is the selection of tires and manufacturers you have in the larger size is larger. With less sidewall "give" AND "bulge" you can run a larger/wider rear tire without concerns for rubbing. This is why the 17s are much wider overall ... they also perform better because they are newer molds then the dusty 15" BFG molds.
As for the rear ... the BMW "open" is not totally "open" and it will in fact act differently than a Ford or other make's open rear. I'll have to find a link to more info ... so many bookmarks, so little time
