When I had a 146 I started using shell optimax (or was it formula shell) then and noticed a difference straight away. I have then had a couple of 156s and a couple of 147s. The difference in these later cars seemed to be slightly less although my 147 2.0TS had the engine management light coming on when I used regular fuel in it a couple of times. Now Im driving a 159 and to start with I would have said that it made next to no difference, but at present I'm using Tesco super unleaded and their £6 off £40 vouchers and I really do think it feels different, just a little more responsive, eager to rev.
Is it worth the extra? well if you bought an alfa for economy then I suppose you're best off sticking to regular, whereas if thats not you then buy super.
You'd save more by just driving more carefully anyway wouldn't you? braking less, smooth take off, using the cruise control.....
I understood that the engine management system managed the ignition so that it maximised the burn of the fuel and that it was only cheap/economy cars where it made no diffeence. I'm casting my mind back to Top Gear/Fifth Gear/Auto Express comparisons a couple of years back. Renault Clio - no difference, Ford Focus, better performance, Subaru Impreza better performance, by about 8 -10bhp. They tried regular, Shell Optimax, BP 'Super' and another one. Correct me if I'm wrong but I think the BP one wasnt too good.