There was a piece done on this a couple of years ago in Sports Compact Car, and Car and Driver. They were looking for horsepower, not mileage. Nothing happened for horsepower.

I had a system called "Direct Hits in my Subaru Outback Sport". Highway cruising mileage under 3500rpm was fantastic. I held the car at 3000rpm, and could get over 33mpg compared to the 27 in normally got at that rpm. The system was finicky, but it did work.