I think I have heard something along the lines of doing resonator deletes, but that's all I know of.
And I wouldn't be surprised if there is an aftermarket exhaust for GT350's. But I'm not sure if it is produce/sold by Ford Performance. I know that in the door jams on my 2017, there is a Ford Performance badge, but I don't know if that translates to saying this car has the Ford Performance parts (exhaust in your case) your speaking of.
Is there a particular website you are seeing advertised? That might be able to help. Or maybe someone with some more knowledge of the subject can chime in.
EDIT: After a few minutes of google searching, the only thing I could find was this:
https://performanceparts.ford.com/part/M-5200-MSS
This is just a replacement of the stock exhaust system that comes with the vehicle. There are also other entries on the Ford Performance Exhaust system in the catalog, but they are all upgrades for the 5.0 engines and lower to get the GT350 active exhaust functionality.