The US rating systems are a bit different than yours.
Here in Arizona, the standard "Premium Unleaded" is 91 octane (RON + MON / 2) with the cars optimized for 93 octane (R+M/2), so on track days, I start with 4 gallons of 100 octane Sunonco Race fuel (available locally for $5.99/gal), then top up with 91 octane to give me 93 octane delivered to the engine. Any higher octane would be a waste, as the chip is optimized for 93. Besides, the higher the octane, the slower the fuel burns, hence the popular misconception that higher octane giving more performance is silly unless the vehicle's timing can be retarded enough to make use of it!

Greg