Same thing happened to my first 280zx. I guess it too spent its time on a slow boat and on the dock in Baltimore. I just kept it waxed and used a bit of fine sandpaper on it. My car was a light blue metallic and I didn't like the idea of having the dealer repaint it. I had it for 3 years and the spot never got worse.
I keep an eye on the rear door of the Xfire. Read here there was also a rubbing problem on the lower part of the car body in that area. There are two hard rubber stoppers on the bottom of the rear hatch that apparently have been known to rub the paint off. I keep a lot of wax in that area even though you can see it when the rear hatch is shut.

Pat