.. Your stock exhaust is all mandrel bent. Replacing it with another mandrel bent exhaust system of the same size is a huge waste of money. Replace the factory muffler and call it a day. Unless you are upgrading to a 4"

Also the drop in air filter and replacing the mid pipe will yield noticeable gains with a tune.

 

muffler and tune can net mid 13's
muffler, tune, lt's, intake/mit tube drop in filter and net even lower on a 2wd ecsb.

 

Mine must be different than, cause its not all mandrel bent

 

I'm only replacing mine because I'm adding another muffler, cutout, and running it out the side before the rear tire

 

What year?

 

Mines a 2012

 

It's hard to tell in this pic, but mine flattens out a few times like this

 

Yours is factory mine looks the same way. Unless you are going turbo or huge FI save money for more useful mods swap out the muffler if you want a little different sound.

 

Lol guys I'm putting 1 7/8 headers with a 3.5 ory pipe, dyna max race bullet infront of a aerochamber and exiting before the rear tire. I'm not trying improve the flow

 

That flat piece in your exhaust isn't going to hurt HP at all...... It still have the same Area as a round pipe, its just flattened to clear the Transmission crossmember

Exhaust Clip - 2011 GMC Sierra 6.2L Pacesetter LT's Magnaflow SI/DO Muffler - YouTube
20-90 Punch - 2011 GMC Sierra 6.2L Tuned 20-90 Roll - YouTube