Interesting, because these engines are optimised for 95 RON.
I did a lengthy mpg comparison many years ago with a 1.8 Civic using Shell, BP and Tesco, using 95, 97/98, and 99 RON depending on what each company supplied.
Same commute roads, same time of day, etc. Taking in motorways, A and B roads.
I could find no driving difference and the mpg increase was tiny < 3mpg, even though, at the time, BP were quoting a large tank range increase.
Since then I've stuck with Tesco 95 without issue.