Do not do a muffler delete. Your ears will be thankful.
I have an 11 GTS 5 speed and only thing I really have "performance" wise is an HPS intake. I've read that with an intake and a tune they can pick up 11 hp. I just haven't justified the tune yet because all my extra money and effort is going into my 3000gt. My friend is a tuner and already told me tuning wouldn't be an issue.