RE: 18 inch stock wheels
The 18 inch wheels look more like the concept car created in 2006 and I think they look superior to the chromed 20 inch wheels. The tires used for the 18 wheels are a superior product (Michelin Pilots).
Performance differences? The 20 inch wheels & tires will be a stiffer ride (less sidewall on the tires) but the tires will wear out pretty fast. Have you priced out a set of 20 inch tires?? You'll go broke keeping those 20 inch Goodyear tires on this car. I do not think that the minimal performance advantage is worth the expense.
Stick with the 18 inch wheels & tire setup - you will not regret it.