I never measured the sound level, but it made a big difference using DynoMax Bullet mufflers on a 3" TTI X-pipe system on my small block.

As for performance difference, it did help my combination despite the added weight. Not more than a tenth, but positive nonetheless. Try jetting 1-2 numbers leaner with the X-pipe vs open headers. But it depends on how close you were to optimum with the open headers to begin with.