I think the AFM would do quite a bit if you are on extended highway cruising with no variable speed changes. Not sure if all the issues with the special lifters and crap for AFM have been resolved so not sure it's worth it or not.
I have a manual tranny so ill see if it comes on during my next highway ride. I don't really care what the gas mileage is since I knew she was gonna be a pig (and she sure is), but if I had an option id actually prefer that the car I bought with a V8 stays just that... a V8.