It's worth keeping a sense of perspective here though!
The unsprung weight on a front corner will be made up of the wheel, tyre, wheel nuts, stub axle, hub, wheelbearings, brake disc, brake calliper (and pads), outer CV joint, hub nut, track rod end, brake backplate, ABS sensor, about half the weight of the anti roll bar drop links, the spring, the moving part of the damper, and about 1/3 of the weight of the wishbones, trackrod, anti-roll bar and driveshaft. At the front that's likely to be a fair chunk of weight - 50+kg???
I have to say that I don't think I'd be able to spot the difference 2kg made in that lot, and if I did, I'd be looking for a nice light set of steel wheels! (and a job with McLaren)!
If reducing the unsprung weight made THAT much difference, there would be a lot more cars around with inboard brakes (the biggest single difference that can reasonably be made). The move towards the use of aluminium has, I think, been as much to do with the manufacturer's obligations for improved recyclability as anything else! When you feel the weight of Alfa's alloy top wishbone assembly it's pretty easy to see that they could have made it in steel - perhaps even lighter but certainly without appreciable weight penalty. At the very least Alfa could fit gas-pressurised dampers "upside-down" to reduce unsprung weight if it was THAT important!
No, My money is firmly on the reduction is sidewall depth being responsible for the poor ride!