Maybe, I was getting around 20mpg out of my TT Malibu so 15mpg on the Cadillac was just a guess since it will be at least 1000lbs heavier.
Either way, not a major concern but I think you would agree it needs a big gas tank
As for color, my gut is telling me black on black (such a nice contrast against all of the chrome and stainless).
May sound a bit boring but I think it keeps with the quiet/classy theme.