I was seriously thinking of the HRE C20 in a 19". This is a very light wheel that should reduce unsprung weight (compared to other 19's) and thus improve handling. They look good too.
One thing to consider is that every pound shaved off of the wheels is equivilant to about 4 pounds elsewhere on the car due to the effects of inertia and centrigual force. If you upsize to 19's, I would do some close checking to ensure that the new set up does not add weight - it seems that a lot of race cars actaully use 18's because they are lighter and stronger. That is why the HRE's were the logical choice.
Considering that the HRE's cost about $7500 per set, I eventually elected to stick with my stock 18's (ball finished with the vintage package). I think that they have a classic look that really complements the car plus their offset and design makes them pretty resistant to bent lips from road irregularities. Our roads here in Calgary are crap so I think going to a lower profile tire would be problematic for ride quality anyways.
If you like the OEM GS wheels, there is a guy out east that sells them refurbished for about $600 each. The only caution is that these probably won't do much for performance as I believe that they are the same width as the OEM 18's but larger diameter (in other words more weight). They will allow you to run a lower profile tire though which will give you a bit sharper turn in (since you are in LA this should not pose a problem).
All of us here who have custom wheels on their Maserati please post pictures of the car and type in the brand name, model and size of the wheels. I was looking at Tire Rack last night for new wheels and kept coming back to IForged wheels for my coupe, but cant' get an idea of what it would all look like.. Any takers on my idea?
I love the selection on this site, but have no idea how they would look!