I could understand cross shopping the BMW and the Infiniti, but when you toss in the other two I get kind of confused. What are you looking for exactly? Could you sum it up in a few sentences?
If you have a car you will DD, and are more or less looking for a weekend or warm weather warrior, get the GT500. If you want something that is more DDable and reliable, go for the G37. If you want something less extreme than the GT500, but with more power options with a few bolt ons, go for the 335. I wouldn't consider the Audi, were I you. Not that it isn't a fantastic car, because it is- I just think it would be somewhat redundant having two sedans when you could possibly pick up one of those svelte coupes instead.