FWIW, I have a 360 with TTI step headers - 1 5/8"-1 3/4" - which I ran open for a couple of years. Then I added a 3" X-pipe and Dynomax Ultraflow mufflers dumping at the rear end housing. I removed the 60lbs the exhaust added to bring it down to the same weight as before and the car was .05-.10 faster!

I figure that the mufflers didn't hurt at all and the X-pipe helped on my particular combo. I may go to their bullet version to save a little weight and gain a little more room around the rear end housing.