I beg to differ. It has been consistently shown in the 6 community that catback modifications on naturally aspirated 6's yield little or no whp gains mostly due to the fact that there are very few flow restrictions from the cat on back. The illustration below is a pic of a Magnaflow cat back. The stock catback is just as free flowing and looks exactly like it save for the mufflers. You'd be hard-pressed to find a straighter exhaust than the stockies.
Most of the restrictions are up from the cat. Exhaust mods on the 6 are generally regarded more as an acoustic modification, not a power modification.