It's called turn down exhaust.
I've heard that it increases noise on classic hot rods by bouncing the exhaust tones off the street.
I didn't realize Till now you can get them for late model rides.
The only advantage would be sound, not performance.
The 05-09 GTs are not that loud to begin with.
All that matters is do you like it. Does it sound good to you?
Curious, what's the rest of your exhaust set up.
08 GT 5spd
Spectra CAI, Comp Cam Detroit Rockers, Kooks headers w/catted H-pipe, Revolution Automotive dyno tuned, FB430 race mufflers, 3.55s, JLT catch can, BMR Lca's and relo brackets. Prothane motor mounts and sway bar bushings.
20% tint, smoked markers and tail lights w/sequentials. Clear front signal covers, series 3 chin spoiler.