I have a Borla Cat back on my 2004 Dale Jr. At idle it is almost the same as stock but it does get louder when you start driving. I'm happy with mine. I still have the stock U-bend with mine. The Borla system comes with a resonator built in. Search for "Borla" in this section and you should find my post about getting mine installed with a couple of pictures.

I think that it actually helped me get better gas mileage because I just came back from Georgia to Missouri on vacation and on the way out there, I got 29mpg for the first time since I had this system installed and since I have owned the car. Last year when I went to Texas on vacation, the best mileage I got was 27mpg.

Wayne