This is something I've experimented with quite a bit. In summary.. if your tune is stock you won't gain anything from the octane alone. The fuel might have more energy in it causing the load to go down. And from that you'll gain a little. But if it's just cheap gas at a higher octane you won't gain a lick.
Now if your tune/timing is adjusted for the appropriate rpms vs load. Then yes you will increase MPG. Since I can tune my own, this is what I've done.
Even though I run shell 93 or sunoco 93/94 costing $0.20 more per gallon. It's cheaper overall because of the increase. Another thing I greatly benfiet from is low end power. I've got lots more timing down low too..