I have Kooks 1⅞ long tube headers with catless mid-pipes. I still have my stock NPP catback exhaust and mufflers. I love the sound and volume of my car, I don't think I'd ever change it. The stock NPP catback already sounds great and is less restrictive than most aftermarket systems. You can find guys on here selling their NPP system all the time, and for cheap! Here's a before and after video I made:
http://youtu.be/0PDuONQ6wTk