In early 2020, Vivid racing (car tuner, Phoenix, AZ area) put together two videos on their Roxor ECU performance tunes and different exhaust configurations: muffler removed, cat removed, two different diameter full length exhausts with muffler, and 2.5" dump and 3" dump pipe into the front fender well.
The result from their dyno runs (3 runs per configuration): removing the stock muffler or replacing it with a higher flow exhaust added single digit HP and torque. Removing the cat or a or adding a bigger pipe than stock made absolutely no difference in performance.
They since removed the video that shows the lack of gains and left the one that shows the video for the Agency Power exhaust they sell. All the video says is that it sounds better. Bigger exhaust on the Roxor is a waste of money with respect to performance gains.
If you want a louder sound (and turbo whistle) for a few bucks do this. Buy a 2.5 inch inlet down pipe.
Remove the 4 foot segment of the exhaust with the muffler (just one clamp to remove). Mount the down pipe and dump the exhaust under the Roxor by the rear passenger tire. Leave the cat alone. Total cost $27. I my Roxors have a metal roof, no enclosure. I can hear the turbo really well (which I like) There is no drone.
I bought this exhaust tip Amazon: LCGP 2.5 Inch Inlet Turn Down Exhaust Tip, 2 1/2 Inch Inside Diameter Inlet, 9’’ Length Stainless Steel Turndown Exhaust Pipe
Caution: the stock muffler is also a spark arrestor. Leave the srock muffler in place if you offroad. If you have a closed cab, keep a full length exhaust to keep fumes out of the cab.