The NPP valves open is pretty much open exhaust from the cats backwards. So if you have cats pretty much all exhaust systems are going to be as loud or quieter than NPP open. They may sound different but not louder.
Remove the rear cats but keep the front ones and it will be louder and wont require a tune.
Fitment is really the only difference between axle back and cat back.
https://youtu.be/EG75jiu9J38?si=W0sipBh0oywLY654
Here is my car, long tubes, no cats, NPP with valves open, for a reference