High octane burns SLOWER than low octane fuel. The reason currnet manufacturers advise the user to go with lower octane fuels is to REDUCE cabon buildup, a by product of combustion. Low octane fuel burns faster and more complete than high octane fuel, therefore there is less carbon build up. The engines today are "mapped" to use the low octane fuel. if you use high octane on a stock engine, you will get carbon buildup which can lead to engine failure after extended use.
Bill Day