In the TS and the V6 I have never noticed a difference no matter what grade I put in or where it comes from with the exception of Morrisons fuel where the car seemed to loose a ton of power.
In the Multiair there was a massive difference. Supermarket basic was a no go. In dynamic it wouldn't generate full boost when going up a hill so the turbo kept dumping pressure. Branded forcourt was a little better but still couldn't get full boost all the time. Supermarket premium and branded forcourt premium was the only fuel where the car performed normally but the MPG was a bit crap on supermarket premium.
Alfa GT 3.2 V6 - Daily Driver.
Alfa 145 QV ph1 in green (Death by rust project) For Sale.
Alfa GTV TS ph2 in blue