i did a muffler delete on my LS3, with a magnaflow x-pipe( part # 10791). All the x-pipe did was reduce the interior sound (which is not loud, no drone) and some of the popping when you decelerate hard- for example, high revs, then take your foot completely off the gas. this only happens at certain rpms, not all the time. this happens even with the stock exhaust in tack, if you have an LS3, you probably have heard this, but it is not as loud. The x-pipe also made it rev a little faster, due to the free er flowing exhaust.
the first pic is of the factory h-pipe that is after the cats and before the resonators.The second pic is the magnaflow x-pipe that replaced it.
there are actually two sets of cats on the LS3. the first video shows a look at the stock exhaust on the LS3.
the second video is the LS3 with the muffler delete
headers will make it alot louder. many aggressive sounding mustangs and camaros usually just have an x-pipe and factory resonators. this mod sounds pretty much like an Axel back exhaust, maybe a little louder. no dash lights or any problems. i don't think headers will void the warranty, as long as you keep all the cats in pace.
this mod cost me about $180, not including the new black chrome tips that were $140.
hope this helps.
this is a video of the delete and x-pipe