I am new here. Just order my X1 with the M sport option, which comes with the staggered 355M wheels.
Even with the xDrive, I think the staggered wheels will help a little bit on acceleration and cornering. Don't forget AWD can't defy physics, when you accelerate, the momentum of the car will shift to the rear. At the same time, the BMW xDrive will transfer most of its torque to the rear axle (behaving just like an RWD in this case). During cornering too, the Performance Control will also kick in and put more power to the outer rear wheel too.
I agree, most of it is just for the look.