I think something has to be said for how comfortable the seats are. I mean, pop your car into SPORT and you'll feel the suspension firm up and the bumps feel more pronounced. However, it remains comfortable and that has to be in part thanks to the seats
EDIT: I should note having had both run flat tyres (on my 116D) and normal tyres (on my M135i)... well, with the low profile both types have reinforced side walls and are at 38 psi but I guess the non run flat tyres seemed a little smoother on the same roads.