I get to drive a friends occasionally, truly amazing car (evo8) just asked him about sticker, his hasn't but has one in filler, it says "use min 98 RON" , to this day he doesn't know how mitsubishi knew his christian name!Most Evo's are meant to be run on super unleaded. Mine had a sticker on the dash saying to use it.
My Fiat coupe 20v Turbo also went better on super unleaded, but it was chipped.
I doubt super will make much difference on an Alfa, they don't run high compression ratios.