My gf and i did a test on our Yaris where we would fill the tanks with 87 or 91 and see if there were any differences in overall MPG.
We do realize that in order for a test to be 100% accurate that we would have to drive the same and take the same routes at the same speed and the same accelerations and stuff all the time.
The yaris was modded I/H/E, are most to note and the way i drive is pretty consistent. Call it biased, call it all in my head or that we drove differently BUT, with 87 we were running between 32-34 mpg average. On 89 we ran 34-37 average. On 91 we ran 36-39 mpg average.
We did this over about 6 months where we filled up 50-60 times overall. My thought is basically that since higher octanes help resist predet which is why it's favored in F/I cars and high compression cars it also burns cleaner under normal circumstances so you get better fuel economy out of it.
With that said, I haven't filled my 2 with premium yet, I'm too frugal and too chinese(penny pincher) to feel the need to pay an extra dollar or 2 per fill up to see if i get better gas mileage. BUT i've been able to reach our higher mark of 35 mpg but i've also been able to reach the lower mark of 29 mpg when i floored it everywhere on regular 87 gas. I average 33mpg overall in my fuel ups so far.