I know that Gibson has a bolt on CAT back kit made for the 05 Frontier. I believe they dyno their system at 11HP and 15 lbs. torque. I just added a Flowmaster system to my 05 NISMO CC. I had to add a resonator because I felt that the cab resonance was too loud. Although, I heard that the Gibson system is not too loud for cab resonance.
You may want too ask some of the other members on this site. I know that there's two with Flowmasters and one with a Gibson. A member name Revrend has the Gibson and he wrote a pretty good review.
By the way, at what RPM does your Frontier idle at? After I added my exhaust system I noticed that it idles at 750 RPM's. I don't remember if that's the same as the stock exhaust.