GM-GMC: CAT back exhaust, chevy silverado z71, flow master exhaust

Question
I have a 2002 Chevy Silverado Z71 ext. cab 4x4 with a 5.3L engine and about 25000 miles.  I had a question about a rumor I heard.  I was about to put a cat-back exhaust on my truck to get a little better performance and better sound, but I was told that with the new Chevy engines that adding an aftermarket exhaust wont make any difference in the performance at all.  In some cases even making the performance worse.  I wanted to know if this was true.  If it is not true then is there any one type of exhaust system that would be best suited to regular driving and medium towing situations?

Answer
Matt,  

  New cars are getting better and better with engine breathing and that includes the exaust. But, theres no question that a lower restiction muffler will increase flow witch is something the industry has to compromise on since most customers want a quiet sounding exaust. I would start with a quality set of low restriction mufflers, any of the major brands are good, I like the sound of a flow master exhaust but thats just personal preference. I have never seen any inprovements in aftermarket catalyst so these I would leave stock. Also I believe your 5.3L uses a 2 in to 1 design, that is the truck uses two converters but then merges into one muffler just past your transfer case. It would be nice to change this to dual exaust sytem but it may not be possible due to a very tight fit on the drivers side by the transfer case, unless it can be split where the two pipes become on at the muffler flange. Check with a compotant exhaust shop that has a good reputation for doing cutom work! Hope this helps!  

****Make sure that the 4 oxygen sensors are not relocated or removed for any reason. All 4 are moitored by the PCM(powertrain controll module) and will set a check engine light if missing. Also its not a good idea to lengthen Oxygen sensor harnests due to calculated resistance values in the wires.******