I was curious about this so I actually experimented with regular vs. premium. I ran spreadsheets (I know, pretty geeky) to calculate my gas mileage based on what octane I was using and where I was buying it, in case that had an impact. Based on almost two years of testing, I'm getting 2-3 MPG higher when I use the 87 octane fuel as opposed to the 91 (or 93 in a couple of cases). I don't notice any appreciable difference in the sound of the engine or the amount of power I'm receiving, but since that's all subjective and the only thing I am actually measuring with any sort of rigour is mileage, I can't say for certain.

I do know that I am running a completely stock engine.