There is even better airflow with the double but the NA can't capitalize on it so you won't see the gains you have going from the stock to single.. Allegedly single vs double makes no difference, but aesthetics wise I think the double looks a lot better so that's what I went with

For the exhaust, search youtube and listen to some examples. Ansa is an option that gives a nice European sound.. other than that, Magnaflow seems to be the most popular