The early GTI's state 98ron on the fuel flap, the later GTI's state 95 ron. Not sure about the ED30 and Pirelli versions.
I have an 07 plate GTI. It states 95ron on the fuel flap and in the manual it states there will be no performance or fuel economy difference using a higher ron.
Maybe there would be a difference on the earlier models?
It's just what the manual states. If a test was done on the mk5 GTI early and late we would find out.