I would venture to say that the majority who have replaced the runflats with non runflats have made at least some suspension modifications. As others have said the stock suspension is soft and set up for the hard run flat tires. In my opinion the stock dampers are the weakest link in the suspension (and the springs as well in pre 2011 model year cars), although there are certainly other parts of the stock suspension that could use improving. I'm also a proponent of the Koni Sport (Yellow) dampers.
For various reasons I didn't want to lower my car so after replacing the runflats with non runflats and larger 19" wheels I changed my springs to 2011 M-Sport springs, which in the rear are linear versus progressive like the 2008-2010 rear springs, and added Koni Sport adjustable dampers. My other suspension mods are all M3 pieces. Those include M3 FSB, front control arms and tension rods and rear guide rods. I next plan to add M3 rear subframe bushings.
My car now rides and drives so much better. Less body roll, better compliance and better handling. And because I went from 18" to 19" wheels it looks lower even though I haven't lowered the ride height.
2009 BMW LeMans Blue 135i/Dinan Stage 2 tune/Alufelgen SF-71s/M3 suspension components/Koni Sport adjustable dampers/ETS FMIC/M3 paddle shifters/Alpina B3 TCU flash