Per Borla's website "Add BORLA® X-Pipe with Mid-Pipes 60512 to create a Cat-Back™ system". With that said I definitely remember seeing catbacks for the GFX cars. I'd say Borla just stopped making them since the 5th gen is getting old. But I found some stock on phastek and summit racing. Phastek is a sponsor of the forum.
Borla S-Type
I'll also add my 2 cents since my car already had a S-type catback when I bought it. With factory cats and manifolds it was too quiet for me even though it had a good sound. I replaced the mufflers with Borla ATAK axle back kit and it's still tame with factory cats and manifolds. Can't wait until I put longtubes on the car.