Originally Posted by nospam
M sport owners with staggered setups should also benefit from more rubber in the back.
All your other points are valid, but this one. Most cars (those that don't have 65% of the weight on the back, ie 911's) use about 90% front brake / 10% rear under full braking, so rear tire width plays nearly zero role in braking. Straight line traction and more understeer are the "benefits" of BMW's staggered tire sizing, but braking advantages are not one of those advantages.
The simple fact is that the US F30 328i and E84 28i are underbraked and have US spec pads that underwhelm. It's certainly helped by grippier tires, but those don't solve the problem, and I'd suspect, on the track, would actually more quickly expose this issue. BMW has gone the route of Honda with their brakes on these two cars.