Muffler design is a compromise between sound level, longevity, physical size, and its impact upon an engine’s horsepower output. Mufflers employing a fibrous packing material for sound deadening have a reputation for high output and reasonable sound level, but the mufflers become louder as the packing material fills with carbon and liquid hydrocarbon particles. Unfortunately mufflers which are both quiet and that have good longevity seem to reduce an engine’s output the most. It is possible to manufacture a muffler that is both quiet and promotes high output ... but it requires more physical size than what is available in a Pantera.
In my opinion the factory GTS mufflers, and those sold by Hall Pantera, are the best because of their sound level, their longevity, and their "factory" appearance. Those are my priorities. However, they are not good choices for an engine having a camshaft with high overlap and/or narrow lobe centers. A camshaft with wide lobe centers, low overlap, and an early opening exhaust valve is better at coping with exhaust back pressure.
If I were to install mufflers using fibrous packing material as a sound deadener I would join the mufflers to the tail pipes with V-band joints, which would facilitate future replacement when they got loud.
I've seen the PPC mufflers on Panteras, always wondered where they were from. Definitely attractive Larry.