Cant comment on the question itself, but more power does not necessarily mean less mpg as some people seem to think. Ive owned many omega, 2.5's and 3.0's and the 3.0's were actually better on fuel. So many things to consider. I used to have a 2.5 vectra sri that could do 50 mpg on a run, no problem at all!

The problem with people quoting mpg figures is that some people try and work it out all wrong. They'll stick a tenner in when the light comes on the do x amount of miles and wait for the light to come on. thats just not the way to do it and the numbers can be way out. Only way to do it is fill it up, zero the trip, then re-fill when nearly empty and work it out properly.