the removal of the cats on a D3 S8 an engine out affair?

Curious minds want to know

The answer is yes. BUT if your keeping your car its not a bad investment. Over time the prone to missfire V10 can do big damage to the cats and then there is the big elephant in the room the 02 sensors.

Some of the missfire issues people are having stem from aged 02 sensors. This is a known issue in the RS4 and a few other FSI motors that they see phantom misfires due to bad or aging 02s.

So while yes you do need to remove the motor for this job it’s not all bad as there is a healthy power pick up and it’s a good step in helping the long term health of the motor itself.

My concern is that the car will be significantly louder. Am I correct?

Well it will be louder but not crazy. It’ll give the car more of a sport sounding exahust. The mufflers and resonators will capture most of the rest of the increased noise.

I have mufflers in place but no resonators. This is not my ideal set up (PO did this) I would have liked it better if the resonators stayed in and the muffler was out. I fear that with the resonators already deleted, if I castrated the cats then it would get obnoxiously loud.