Stillen CAI gives it a nice sound, very little if any HP gain and only at high RPMs.
Berk technology makes a catback and high flow cat combo that has been shown to give a good HP bump. From what I am told its the highflow cats that give it the biggest part of the gains. I love the sound of this exhaust as well. It's quiet at low RPMs/street driving, and sounds very good when at the track and running at higher RPM's. I will buy mine if they ever come back in stock
Don't do headers unless you can do the install yourself. Labor is >$600.00 because the headers are not an easy swap.