Originally Posted by dnvrdrvr
when I had decided to go square with these at 10"...
What tires are you using? Track temps, session lengths, etc?
Having a square setup does not decrease grip on the rear, it simply improves front grip a lot more. The rear feeling loose is simply the car being over driven because of the amount of front grip available. If you were in a staggered setup with the same sized rear tire as the square setup you are looking at, you would not be faster. The rear grip would be the same, and the square setup would have more front grip. You'd need lap time data to really see this, but I would not turn down front grip if I can fit it.
Most everyone I know tracks their car with a square setup (e30 to e90). Even a 275 is a lot of rear tire, especially if it's an r-compound. The one E90 m3 that competed in the 25 hours of thunderhill ran a square setup. And two other BMW's that ran our APEX wheels in that even were on 10" square setups as well.
Staggered setups on this car will not have the oversteer feeling of a square setup, but you will not be able to rotate your tires, which can be costly for some.