I put 91 in at my last fill up. I couldn't believe the price difference between 87 and 91. I'll be filling up again today but will go back to 87. I don't know if it was just butt dyno or what, but the engine seemed a bit more lively on the 91.
There was one improvement that I know for a fact, however. On 87 every day when approaching the highway on ramp at around 45-50mph, there's a noticeable RPM surge with the motor under load. It'll rev a little higher, drop back down, and then rev up again. You can watch the needle move. On 91 that occurrence seems to have disappeared.