I think I get 16-17 city and about 20 highway in my VX. Thought it was bad, but reasonable for about 245HP I'm supposed to have with the intake/exhaust upgrade (compared to a BMW 525i with 220HP, getting 18/24).
I hope I do not see a decline like you are describing, I'd expect 11mpg on a V8. I noticed decline driving on a highway in the Power Mode.