Assuming that the stock mufflers are the same as those on the G8s, they are no detrament to performance...guys have gone into the 12s with stock GP mufflers. The piping from the manifolds to the mufflers is far more important. Here's why:
The volume of exhaust gas is highest closest to the engine because it is hottest there. As the gas moves through the exhaust system it cools, and by the time it gets to the mufflers at the very end of the system the temperature and thus the volume are much lower. Result - piping at the end of the system is not that important to gas flow restriction.
The sound is a different matter of course, and that is the real value of mufflers on the GP - IMO anyway. While I like the stock '04 GTP mufflers fine, if I wanted more sound I'd make sure to get a muffler with packing in it, not a chambered muffler like FloMasters ( they really should be called
SoundMasters ). On engines with less than 8 cylinders, chambered mufflers just sound bad to me
. My blown Dakota R/T has a Dynaflow muffler ( came stock ) and I like the sound - doesn't sound like every other truck running the ubiquitous FlowMasters.....