I've used a similar "muffler" from Summit designed to be placed in the reducer for people who have open header cars, and want to stop eating Advil....
They're about as effective as 1-Chamber Flowmasters; or at least they were on my Nova... If you're running anything quieter than that, you'd notice a big noise difference.
I am, however, thinking of putting them in conjunction with my 3-Chamber Flowmasters I've got now; they mellow the sound, offer little restriction, and do quiet it a bit.
Do these install with a pop-rivet/weld?
-Andy/Oldimpala