don't know about the back pressure, but i drove mine open headers basically, to get my h-pipe modified to fit on right....sounded loud and tinney,didn't notice any difference in the car aside from the noise. but maybe it would sound a little different running through the tail pipes??
my friend had straight pipes well no mufflers but kept his cats and it souned pretty nasty..... all you can do to make a v6 sound better is get a set of mellow low tone mufflers and live with it taking the cats off or running no mufflers sounds VERY bad.
Warlocks. They are the best sounding muffler for our cars. Period. Hands down. Any other muffler need not apply. They will make your eyes, ears, and throat water. Gaston ran .2 faster just by uncapping them. They own.
But whatever, go with your POS bassani or flowmasters that provide nothing but nostalgic value.