Higher octane fuels give more power

On modern engines with sophisticated engine management systems, the engine can operate efficiently on fuels of a wider range of octane rating, but there remains an optimum octane for the engine under specific driving conditions.

Older cars without such systems are more restricted in their choice of fuel, as the engine can not automatically adjust to accommodate lower octane fuel.

Because knock is so destructive, owners of older cars must use fuel that will not knock under the most demanding conditions they encounter, and must continue to use that fuel, even if they only occasionally require the octane.

If you are already using the proper octane fuel, you will not obtain morepower from higher octane fuels. The engine will be already operating at optimum settings, and a higher octane should have no effect on the management system.

Your driveability and fuel economy will remain the same. The higher octane fuel costs more, so you are just throwing money away. If you are already using a fuel with an octane rating slightly below the optimum, then using a higher octane fuel will cause the engine management system to move to the optimum settings, possibly resulting in both increased power and improved fuel economy.

You may be able to change octanes between seasons ( reduce octane in winter ) to obtain the most cost-effective fuel without loss of driveability. Once you have identified the fuel that keeps the engine at optimum settings, there is no advantage in moving to an even higher octane fuel.

The manufacturer's recommendation is conservative, so you may be able to carefully reduce the fuel octane. The penalty for getting it badly wrong, and not realising that you have, could be expensive engine damage.