98 octane fuel has been refined more than lower octane fuels. Per unit, 98 octane has a higher efficiency, and this is what equates to the power difference and extra mileage. If your vehicle is tuned for 98 octane you will see the full benefit, but, on cars like mine where the ECU uses a 'range', the range will need to be remapped to get the full benefit. Old cars with a distributor simply have to advance their timing a few degrees to get the full benefit.
I don't know what you guys have over there, but, we have another type... Ethanol... E10, E15, E85 whatever you want to call it.... This has a higher octane (100-104) then 98 petrol, and is cheaper, but, it's not as efficient per unit as 98 octane petrol. The effect this has is you use slightly more Ethanol in comparrison to 98 octane, so you get to watch your fuel gauge go down and the difference in pricing get eaten up due to using more.