Not much point really I am afraid. wont make the car go better.
The RON figure is the petrols ability to not combust early due to compression.
It isn't a measure of how well the petrol burns, or how much energy can be extracted.

Mk2 GTI designed for 98 RON.
Does everyone else agree with this? Im no experta and dont fully understand RON ratings but i do know that if i put anything other than optimax or equivelent in then i get pinking real bad.
Aye tis true

The RON rating is just a measure of how resistant the fuel is to pre-detonation.
In no way is it a max power type figure relating to how much much power can be extracted from the fuel, (just a piece of useless info for ya: the petrol engine wastes something like 75% of the energy present in its fuel anyway).
Therefore a gti designed to run on 98RON will not run any better on 99RON with the standard settings. However I bet tescos 99 will be cheaper than optimax.