Congrats! Nice color.
I have the Ford long tube headers, and the same Ford (Borla) "muffler". This combo sounds great!
I have heard every combination of headers, cat delete pipes, different mufflers, x-pipes, h-pipes, you name it. I've driven many and ridden in the others. Heard them from the inside and from the outside, on the street and on the track. I have heard it all. And on a non-turbo, supercharged Ford GT, the best sounding combination is the one above. It is the ONLY reasonably loud, really bitchin' sounding exhaust that does not have any drone, at any RPM, and sweet sound at every RPM. If my GT was not turbo-charged this is the combo I would have.
Owners in emissions inspection states that must have cats are out of luck as the FRP headers feed directly into the Borla and there is no way to have cats. And this combo does not work on turbo GTs either.
You will hear some owners say that their X-pipe does not drone, but trust me, yes it does. As you already have 1/2 of the magic combo I suggest you try the cat delete pipe and see what you think. If you like it better but it's "not quite there", then put the FRP headers on with your Borla. You can thank me later. Cheers.
Chip