Rear seat head and legroom were big factors for me at 6'4" as well. I really liked the Gran Prix but couldn't sit upright in the rear seat.
Ours in V6 (not GT) and so we have Uniroyal tires (not my personal favorite) but they have exhibited none of the flat spotting you mention.
I also agree that OnStar is over priced for our uses and we don't have it. I wonder how effective the telephone feature is? One of the ads used to be of someone who lived in a remote area and used it because it could get a signal where the larger carriers could not.
Anyhow, good luck with it and welcome.