Due to the fastburn head design for the LSx engines and even stock CATs today the flow is pretty good and not a lot of backpressure
So mufflers themselves will not produce more HP and brand/type of muffler is more about the quality of sound the owner likes.
What can be an issue is seeing the amount of carbon build up on your valves and piston tops that
1. past owner was lazy and never did common maintenance in using a good decarb cleaner
2. That tune and mods were not matched well forcing a too lean of condition that the PCM non stop could not correct the AFR and thus was dumping in more fuel when it was not needed and causing CATS to run too hot causing the PCM to trigger the CAT overtemp protection which was dumping in even more fuel to try and cool CATs down
End result of that along with carbon build up is caking up the CATs, (they might have even been breaking up and clogging the mufflers) and corrupting the O2 sensors.
As much as the junk said in cyberspace USE also the rear 02 sensors even if not using CATs as the GM PCMs use the rears for short term fuel trims and what average fuel injector pulse width ON times for each drive cycle.
You would be real wise at this point to make the effort and verify 02s, CATs and existing mufflers are in good working condition and best is to replace at least both front 02s.
If your looking for gains replace both CATS with a true X-pipe at that location thus existing exhaust pipes and catback can be used.
This will increase the low end torque and change the sound of exhaust.
As to catback change the thought would be weight saving as stock system is about 75 lbs and other designs can cut that weight in half.
Rev limiter reduction is not the fix, the path is use a pushrod length tester to assure proper length rods are used and better vale springs
The later beehive designs work well
Dual spring is more about harmonics of the thin inner spring and not as much to prevent valve float.
If you want to prevent end cylinders from being too hot/lean the coolant lines the LS1 had do just that by the flowing water pulls heat out or
install water methanol injection even with a NA engine will pull 40 plus temp degrees out of cylinders and is cheap in long term costs.
Decent kits are less then $500 and trigger off the MAF output signal so can be adjusted to flow only when in upper engine loads
I understand what you're saying. Unfortunately, all of the pistons have been kissed by the valves some more than others.
So that's telling me that not only did the tuner turn off all of the warning codes but the RPMs were also too high for the springs in the valve train. Me and my buddy are going to try to determine the lift and duration of my Cam and make appropriate changes to the valvetrain if necessary.
I am also planning on bringing my fuel injectors to my tuner and having him pump them up to 36 pounds, after everything is put back together. I certainly don't want a repeat of this disaster.
On a different note. I really would like to pull the Billy boat bullets off my car as they are way too loud. If I put the stock titanium system on my Z would that caused a problem with airflow since I have long tube headers, high flow cats, and that high duration and lift cam?