When I autocross my 240sx at SCCA events, I run my tank almost dry prior to the event, then put in about 1/4 tank of 100 octane on my way to the race. There's 2 stations within 5 miles of me that sell it - used to be about $5/gallon, now it's up to $7.50. The car does perform better with the 100 than with the 91 octane premium that is available. It's a seat of the pants feeling, but it feels like the difference between running with a/c on or a/c off. It's way too expensive to run it on a regular basis, but when you're racing, every 10th of a second counts.

I had tried filling a gas can with 3 gallons and adding it at the event, but didn't acheive the same results. The computer apparently needs some driving time to realize that it can safely advance the timing beyond it's normal driving parameters. I've been meaning to try disconnecting the battery & resetting the computer at the gas station, but I never seem to have enough time.