After running TTI 1 5/8"-1 3/4" step headers for a year or two with just collectors, I installed their 3" X-pipes ending with Dynomax Ultraflows at the rear housing. Overall, it added about 70 lbs. But after getting the overall weight back down and a little carb rejetting from the best I was able to get before, the car was at least .05 faster, possibly closer to .10. So it all stayed on despite being inconvenient for trans or center section swaps. I may eventually go to the Dynomax bullet style to save a little weight and gain a little room around the rear end yoke. (That will allow me to use more easily removable weight if I need move to the next lighter class.)

I didn't do back-to-backs afterwards, but I felt that the mufflers didn't hurt at all and that the X-pipe definitely helped. I can't help but think that each side actually helps scavenging from the other side through the "X" with the alternating pulses. (Just speculating why it helps. I guess basically what topside said.) I installed them as close as I could to the headers with the "X" directly under the very end of my 904 trans.

It works for Nascar! But each combo will have it's own results and may require a little jet tuning for best results.