You may not like my potential cause of the alignment issue, but I had the same problem on my S90. Pulled to the right, had the alignment rechecked twice, no improvement. Everything looked right. Problem ended up to be the steering rack. It was replaced (thankfully under warranty) and the problem went away. No other normal steering rack symptoms, like grinding and moaning. I'll mention this as information that might be useful to you, and then cross my fingers that this isn't your problem!

I agree with TRM's post on handling. This is not a performance car, and it doesn't have any real aftermarket support to improve performance. While the handling is certainly adequate, it's far from spectacular, and in my experience the FWD series are much better tuned. My recommendation: Enjoy the comfy seats, turn on the stereo, open the sunroof, and sit back and enjoy this car for what it is. The only other suggestion I can offer is Dunlop Sport A2's, which IMHO are heads and shoulders better than Michelins.