I think some of that has to do w/ the LS1 - IMO the LS1 was just a mean sounding motor. I've heard identical setups on LS2 and LS3 and LT1 cars and didn't sound as good as LS1's....maybe I'm delusional but I swear they sounded different, more "muscle", more uneven etc. I am pretty sure the LS1 that was in the GTO was slightly higher rated than any other application and also that it may have had a slightly different cam? That may be hearsay but pretty sure it was rated higher than the others anyhow.
My setup on my GTO sounded NASTY and I wasn't even running headers - LS6 heads, AEM intake, stock manifolds, but from cats back was 2.5" straight pipe, H pipe, leading into 18" sweet thunder chambered mufflers - and man I tell you it popped, burbled,gargled and flat out sounded mean
'08 GT sport/premium, tints 20/35, 18" TSW nogaros w/ bridgestone blizzak winter, 19" sport oem w/ hankook evo summer, Philips crystalvision bulbs, debadged, Vararam OTR, JBA 1-3/4" LT's, ported TB, MF axlebacks, MF X-pipe, DBA-powerslot brakes, russel SS lines, hawk ceramic pads, GXP shocks/struts, lovells poly strut mounts, Slowhawk Tuned 344/356
Gone -'04 GTO A4 2.5" custom exhaust, sweet thunder muffs, H pipe, Z06 heads, Baer brake it, 20% tints, torrid red, AEM brute force intake, tuned