I don't know what kind of cornering you do, but I corner the **** out of this car and the stock 4.5s are NOT bad at all. With that said, x7 is killing it. I put on some 14x6 today and just a little more confident than the 4.5s.
Unless you do a full suspension overhaul, and do autocrosses or road racing (hell, even then) I doubt you'll get the full use out of that x7. In fact, I bet your acceleration will hurt more than the benefit you're going to get around the corners.
NOT to mention that the widest possible tires you can put on these cars is probably a 195. Guess what, a 195 can fit in a 5.5