Here is data from our test a few years ago. Note sound levels with and without X.
About 75% were lower with the X.
Air flow is also something to note. For folks that like the chambered muffler sound, you can see they tend to be louder and on top of that, they do not pass near as much air as straight through mufflers, and it makes sense of course, why that is. When you compare the numbers though it is eye opening.
Look, even the old 1965 Corvair Turbo muffler is quieter and flowed more air than some of today's favorites. It is easy to see why they worked so well compared to stockers.
Tom