obviously 20s brings more compromise than say 18s or 19s. but if you're willing to accept these, no probs, go for it!
not everyone places acceleration, suspension control, and ride comfort at the top of their priorities. instead, some bling bling is not a bad thing. however, upsizing would make new, better dampers a good idea, rather than the factory ones; so it's worth planning to change to KONI Sport or Bilstein at some stage if you can.
if you want 19s, keep searching. whilst i can't recommend a wheel off the top of my head, it's worth being patient to find the exact size and style wheel that suits your needs. check through the websites for the major brands, including OZ Racing, BBS, MOMO, etc. there may be something that you like and fits the car.
of course, finding a wheel that fits straight on is always a challenge with alfa's. bolt pattern, offset, brembo clearance, centre bore size, etc.
it took me over 6 months to finally get wheels onto my car. but it was worth it in the end.