I've just used a bottle of the Nulon Pro-Strength Octane Booster over two tanks while travelling about 600kms north of Perth. I couldn't always get 98RON so added half a bottle to a tank of 95RON. The car performed no differently to a normal tank of Caltex 98RON or BP Ultimate 98.
I think it's a good backup to have a bottle in the car in the event you can't get 95RON or 98RON. Faced with the certainty of detonation (knocking) in the cylinders running on 91RON fuel, I'd opt for 91RON + Octane Booster and run the 'perceived' risk with it.