I have an 08 Gt with JBA lt headers and catted h pipe. I had Flowmaster outlaws on prior and like the volume and tone. After the headers though my car was super loud even when cruising at low rpm's. So i re-installed the stock mufflers to prevent from getting an tickets. But now the car is quieter than with just outlaws installed. So if I had these welded on to the down pipes after my h pipe, and put the outlaws back on would they tone it down enough for cruising around town but still scream when at wot?
I actually did what you are suggesting on a prior car to cut down on rasp. It did cut down on the rasp, but overall, the volume was the same. I would only do these if you are trying to deepen the tone. Volume change will be minimal.