Nope I have never seen any 20"s on these cars in the metal. The only ones I have seen online that look decent are the 20" Novitecs on that black spider, they are quite stylish I thought. All the ones on that German website looked terrible IMO, and the Mexican one was definitely too bling bling - most of the Germans too. The 21s just don't seem to work at all.
It is hard to improve on the Alfa stock range I guess! Your tyre costs would go through the roof I think - it is hard to get 19"s in the right size for less than £200 each, imagine the cost of 20"s!!
Also your insurance would be invalidated unless you declared it to them, and it's not the sort of mod that goes unnoticed unfortunately. It would probably bump your premium up by 2 figures depending on insurer, not horrendous but a nuisance to administrate...
Brat Pack no.8