Nope, most of us run on what it says in the American manual which is either 91 (usually found at higher elevations) or 93 AKI octane (so 95/98 for you).
Anyway, you should absolutely get better mileage post-tune - just you might not gain anything over us tuned Americans performance wise since your octanes aren't actually very different. My N/A gained ~3-4mpg on the highway and a few miles to a tank in the city as well. I don't know what the Eurocharged guys do (that's why I pay them
) but they definitely improve mileage with a tune.