The idea here is not that all cars would work better on 91 octane. The idea is that if you raise the minimum octane, engineers can do more to design future engines for better fuel economy. Right now they're limited by the fact that so many people just throw regular gas in there regardless of what the vehicle calls for. They have to design their engines to tolerate lower octane fuel. If they didn't have to worry about that, they could use higher compression ratios, more boost, and tunes that spend less time with rich air-fuel mixtures. That means they can reduce displacement even further for better fuel economy.
Ironically, the best technology for improving ICE efficiency right now -- Mazda's SCCI -- runs better with LOWER octane fuel than we can get right now. Guess the industry would rather just continue downsizing, downspeeding, and boosting like crazy. Who needs broad efficiency bands and good throttle response when you can just optimize like crazy for a tiny range of conditions that almost no one will ever drive in, right?