Regarding the interstate steering questions, how are the tires? When I bought my 2002 S, I thought the tires were ok, but a shop told me otherwise. And I was having very similar high speed concerns.
I was actually amazed at how much new tires transformed the whole car. It got a lot smoother over bumps, cornered a lot better, and handled high speed like a.... Well, like a Porsche!
Good luck!