Racer,
I now have Style 74 Star 18" wheels on my X5. I tried to order them when I speced out my 3.0 as I have always liked their look. Unfortunately BMW will not allow these requests. The ride is firmer but very acceptable. I am not really a "spirited driver" so cannot give you much info there. The 17" do have deeper sidewalls and a somewhat cushioned feel. I really am enjoying my X5 and have not had any issues thus far. Best Wishes.