I wonder if there is an element of "Emperors' new clothes" about using higher octane fuel in cars with modern engines.

There was a comprehensive test done on the higher grade fuels on one of the motoring programmes (fifth gear I think) a couple of years ago.

Have a look at the programme excerpt on youtube

https://www.youtube.com/watch?v=d8L-X89duEs

The improvement in performance from the best of them was at best marginal and some showed no difference.

Results on older engines might of course be different