Aesir,
When I lived in AZ, I mixed 100 octane exactly ONCE (Texaco on Bell Rd). I added 2 gallons($10) and then topped up the tank with 91. I couldn't tell the difference, so I decided to skip the hassle.
If I were going to do 1/4 mile runs, I MIGHT try 100 octane, but I don't do that kind of thing to my car.
P.S> linnig in NY, now I just buy 93/94 octane
at the pump