Flash to 91 when you need it. Never run lower octane than what you are tuned for.
Up until it pulls enough timing it'll knock like a mofo and you'll fail a ringland.What happens worst case? I have a pro tune so can't really dump in a 91 map unless I use the stock one I guess. I know changing the map is the best idea but I think its important to understand why things behave the way they do so you can learn. This is a good question and I'd like to learn a bit.
I'm thinking the car would pull timing for knock reduction?
Can anything really bad happen, or will the car just run poorly? (Assuming this is just for 1-2 tanks)
Is there a valid octane boosting additive available that works?
^^ Agreed. Even running 93 octane on 93 octane tune, I dump 1 gallon of "dirty" gas (pumps around me use one hose for all octanes) into a gas jug I keep with me, and add a gallon of Tolulene per fill up. Jug gets emptied into my DD, so i'm not out anything there.Drive like a grandpa and it won't matter. Worst case scenario, you beat the shit out of the car at a high elevation with a fuel system that is already inadequate while using a fuel of less octane, and end up cracking a ringland.
Fuel additives won't make enough of a difference, but if you want to make sure she's safe and still beat the piss out of her, just bring a jug of race gas and add a couple gallons to each 91 tank.