Went on a fairly long (1000 mile +)road trip last week in Northern Michigan and I had searched pure-gas.org for stations with Ethanol Free gas. Most of it was premium so I was getting ethanol free and higher octane compared to the 87 E10 I have run exclusively for the first 11,000 miles. I did a tank of Marathon 90 octane (they called it "Recreational Fuel"), then the next fill-up was Shell V-Power 91, also E0.
Most of the driving was on rural 2-lane highways with a different weather, and levels of hills, and shorter 70-100 mile runs. So, it was tough to do much highly scientific testing, but I do have some observations.
On straight flat, 55MPH runs, I did not detect and noticeable increase in MPG. It was 40-41 which I have achieved several times before with good weather. What was noticeably different was when I punched the accelerator to pass grandma going 51 in front of me or accelerating up a hill (which I had several in the Upper peninsula of Michigan). I noticed two things. One was better responsiveness to the pedal on the floor and it seemed smoother & quieter at high RPM. The other observation was that these aggressive driving moves did not seem to plummet the MPG meter near as quickly. I am continually hitting the MPG reset on trip 2 so that I can more easily see the MPG effects of different behavior (I need to get a scanguage). I did far more aggressive driving this trip and my numbers were as good as previous trips that were more strict Hypermiler oriented.
Most of my road-trips are usually just my wife & I but this trip, I also had two teens and their luggage so I was about 450 pounds heavier than usual. Even with that added weight, I got 36 and 37 MPG hand calculated tank totals. So, not a lot of controlled science here but my butt-dyno tells me that high-end gas does improve power. Most agree premium results in more power. So, with more aggressive driving you should theoretically be able to do more acceleration with less pedal push = better MPG on the spirited sessions. I'm not convinced the premium would be a financially advantageous (or even break even) move. But the engine did have a noticeable improvement in stress situations.