Bigger front tires don't really help braking and turn-in, they actually hurt it some, but they do help mid-corner and track-out grip. A wider contact patch doesn't change lateral direction as easily as a narrower one. Similarly, a contact patch with a "longer" shape tends to resist longitudinal forces better. Like everything there's no free lunch and the tradeoff for more grip in the corner is a little less steering response and a little worse braking grip. It seems counterintuitive but when you change the tire size all you're really changing is the shape of the contact patch, assuming you're running roughly the same tire pressure and dynamic corner weights don't change drastically. Some cars like ours need it, some cars like 911's and whatnot don't because they don't carry much weight up there. No reason you can't run a 275 with a much wider rear, there's room. The only worthwhile reason for running a square setup IMO is tire rotation and it's not a bad reason at all, but I think it sacrifices some performance against, say, a 285/315 or 275/295 setup if you compensate for the balance changes.
In any case, the only way to go faster is getting the balance right for each individual setup and then finding out which one the stopwatch says is better. IMO One of the reasons square setups are so popular is because this car and BMW's in general tend to understeer with the stock setup so you bolt on more neutral balance when you bolt on a square setup.
To answer your question, I think you'd be happy with the staggered R1's as long as you compensate for the balance changes. They're going to be quite a bit grippier which will add to the sensation of a "tight" setup.
If it were me I'd just run the 285/30-18's all around, the next size up is the 335/30-18 which is probably too big without some serious finagling. I'm going to do a fairly comprehensive study of Hoosier fitment in the rear of this thing this winter and I'll post my results, the purple crack is somewhat similar to the R1 in terms of fitment
Last edited by Richbot; 11-28-2011 at 12:15 PM.