Hello. I have a 2013 LS3 with the two sets of cats. I hollowed out both the pre cats right after the manifold and the post cats in the mid pipe and used my tuning software to turn off the second O2 sensor.. I did this for the more raw sound and to keep the second set of cats from slowly clogging and heating up my center console due to a lot of track use. What I found was having two large volume changes in the exhaust in series caused a strange full throttle shifting exhaust pop and not quite as much rawness in the tone. I solved this by inserting a 12.75" long, 18 gauge 2.375" pipe in the second set of cats as it is a straight run right thru the cat. I expanded the pipe once it barely slid in and it fits perfectly with no gaps. Now the sound is great and the engine seems much happier. I still have one large volume in the pre cats but with no second large volume right after it, it runs great and sounds great and there is no heat build up. I may have actually picked up a horsepower or two as I seem to be pulling a few more RPMs on the straight aways. The oil temp seemed to also drop maybe a few degrees by the engine not pushing thru cats. Here's a picture of the pipe and it installed.