I tend to avoid 95 RON fuel from supermarkets as I have heard to reduce price they tend to use fuel with less cleaning additives but its probably just as good as brands like BP and Shell for all I know.
I recently developed a new bout of paranoia and decided if I must fill up at a supermarket then 98 might be 'cleaner' than ultra cheap 95.. I don't know if there's any truth in that!
Bike just has cheapest 95 as it's still running carbs and doesn't have a lambda sensor to get gummed up (or whatever it was that killed loads of cars a few years ago..)
None of the Alfa engines are highly enough tuned as standard to require more than 95. Would be a waste to put anything better in.
I've had a couple of cars with highly strung engines needing super unleaded. Quickly annoys, especially when the MPG's are poor. Getting rid of my M3 (and get a 147) because of the economy. Getting 17mpg average out of it and paying 117.9 for Tesco 99 ron. Probably works out cheaper to take a taxi somewhere.