A modded Viggen???
I don't mean to sound like The Voice of Doom, but with that level of modification, T5 or T7, you should be running 93 octane at sea level (91 at altitude...). Always. Everyday. Twice on Sunday.
Nearly every tuner derives some of the HP gain by octane optimization -- essentially turning off the ability of the ECU to compensate for varying grades of gasoline. I'd be surprised if Nordic didn't do this (you can email Magnus or Rolf to confirm...or Nick might know of the top of head)
Toluene is what's commonly used in the commercial octane boosters, so it's safe to use in the relatively low concentrations needed for you. I think there are some common formulas to calculate how much to add to X gallons of 89 octane to get a tank of 93...