Non run flat tires have an impact in performance. The rear may swing a lil bit more; some may say that that is technically irrelavant or impossible, and yet, that's what I found out after equiping my 530d with non RFT tires (having it equipped with RFT previously). I believe that the car iis made for RFT tires...! Also, perhaps the difference on the "outer strip" that is a characteristic of the RFT may cause that feeling.
my 2 cents!