Disclaimer, YMMV! Been a hybrider for 8 years, so know how NOT to break the egg.
Back in 2012 had a 2012 F150 Lariat 4x4 Max Tow EB, Loaded. It had the ubiquitous Shudder, Stutter crap and suffered from poor gas mileage. I expected to get 18 and got 16, while the EB Flex SEL that I traded in for it got 21-24 MPG with AWD. Of course Ford could not fix the shudder, stutter adn thankfully never encountered the fall on its face on the highway others have had, however their answer to the poor MPG, use Premium. I asked, where in the manual does it say that, it doesn't, only for heavy use or towing performance. Well I was skeptical, but tried it anyway and indeed it did climb up, to 19 MPG. Got 24 on the Interstate too. With the shudder, stutter adn other issues I didn't keep it(now I wish I did).
Fast forward 3 years, and several cars later I now have a new 2014 F150 Lariat, not fully loaded, 1 wheel drive, but I love it! I ran it for the first 5000 miles on regular, including several hundred towing a large enclosed trailer on the Interstate. After the first oil change and tire rotation, I switched to Ethanol free BP premium, in fact all gas but the first tank has been BP, most from the same pump, so consistent fills, and gas quality.
Using the 87 regular BP with 10% crap in it, I was averaging about 19 MPG with 120 mile DD on 60 MPH highways.
Using E-Free premium for about 1800 miles, the first thing I noticed is pulling power towing the trailer has increased, No surprise, but the MPG while towing also went up to about 17 from 12. That was a surprise. The average MPG though also went up to 21. To verify the results the last three tanks have been regular and the MPG dropped down to around 19 again, so next tank, back to premium.
I haven't done the math for the 700 miles per week I put on Premium vs regular at .40c a gallon more for a 2 or so MPG increase, but it does feel good seeing 21+ MPG on the display. I use Fuelly to calculate true MPG and almost every fill under premium was about 20.8 MPG. When I had the 2012 though the difference in MPG I was seeing actually made using Premium less expensive overall. Most likely in the long run it will pay off, especially if I get a MPG tune for Premium that boost MPG even higher, and if swapping out the gearing for 3:31 or 3:55 for the 3:15 when I go from one wheel drive to two wheel drive(posi install).
Having an increase in one vehicle was a fluke, but to be able to repeat it in another one makes me think that in some vehicles using Premium actually saves money in the long run.