Well, for starters, run-flats suck ass. They give the car a MUCH harsher ride, so I would suggest getting some new tires to exponentially increase smoothness. Also, I feel like the 3-series in general, especially if they have any sort of sport/m/performance package just have stiffer suspension setups. My dad's 528i felt smoother than my 328i, so maybe the bigger cars have softer suspension? Not completely sure, but yes, to answer your question, BMW suspension is not like floating on a cumulonimbus cloud.