firstly i would say that some people who cant see the difference are true, different RON wont make a massive difference in N/a cars,
where the difference is , is performance turbo cars, specially ones running higher boost, the theory goes the higher the octane the less likely pinking and detonation will occur,
in my 5 running 16psi, there is a massive difference between 95 ron, and esso SUL,
also some points id like to clear up about optimax, it is not refined at 98 RON, it is 95 RON that is boost to 98RON, and after a while, the octane rating starts to drop off, which is why some people say with imprezas noticed there car missing if it had been a weekend toy and sitting in a garage, also depends on how long the fuel has been at the station, when it was delivered etc. When its good its really good, but i have had tanks where there has been a noticable difference in the responces of the car (fuel is not just fuel!, in the same way oil is not just oil!)
Esso and BP refined there super unleaded at 97RON, so it will stay at 97RON for a hell of a lot longer than optimax will stay at 98, ive never had anything but superb results from esso sul, and bp ultimate is good also.
Esso now sell 98RON SUL, now i think that is boosted 97RON petrol but over time will be more potent that optimax, i cant comment on how clean they leave the engine over optimax though!
its a case of picking the right fuel for the right engine,