Ride quality is something very difficult to answer. What may be soft and comfortable to one person may be living hell to another, and so on. Stick with the stock tire size. The most comfortable tire we used so far has been the Pirelli P Zero Rosso for you paticular car. Yes, there are other tires out there but I know these tire have a better than factory load rating than you currently have. Thus, according to BMW they are safer. Another tire brand with less of a road rating may be smoother but I cannot recommend a tire that has less than a factory load rating. PS. Even if the load rating isn't as high, I would personally put Toyo Versados on the car if it were mine.