I would go with the black interior... but that is my preferance. I know how you feel, I went from a 750hp Mustang to 220hp Fusion before I got the CT. I still miss the power but dont miss the 10mpg.
They just got white with black leather in stock. Might jump all over it.I toyed with the idea of the water-gray interior. However, the particular color combination that I wanted (Nebula Gray/Water Grey Leather) while theoretically possible, is non-existent in the real world (at least in California). It was actually kind of difficult to track down ANY leather interior. When one came in in Nebula Gray/Black Leather, I jumped on it. Just my experience, but you might be waiting awhile for a water gray interior.
That's one hell of a change! You'll enjoy the MPG and the less $$$, but overall the CTS-V is WAAAAAAAAAAAY more fun (My friend has a '11 coupe)