I agree with BT52b, The drag is proportional to the speed squared. Even if the engine is working more efficiently on slight boost (i.e. less petrol per BHP produced) a lot more BHP is required to maintain the higher speed, this would outweight the efficiency improvement meaning more petrol per mile despite less petrol per BHP.

Another point.

I also get lower MPG on Optimax than standard Unleaded!!!????

I've just come back from Wales put Optimax in on the way there, filled up from a totally empty tank. I got 350 miles out of it. I've got a UK300 which I think is a 60 litre tank, and I was stuck in a lot of traffic on the way there.

Well I tried it on the way home and I certainly couldn't hold my car with some positive boost pressure at 70-80 without it accelerating - it was quite a way below 0 on the boost guage to hold a steady speed - so I don't really understand that point.

As for the fuel consumption being better at this speed, i am sure that I have heard that it isn't from somewhere but if people say on here that it is better then I can't argue. I will try it and see! :)