Audi S6 2002 V8 4.2L
The catalytic converter is empty from inside. The small parts have been stuck at the end of the exhaust pipe and so we cleaned it up.
When I go beyond 140km/hr, I feel like the acceleration isn't as good as it should be. Especially from 180 and onwards, it takes ages to reach 240. The car is meant to reach 300km/hr easely. The catalytic converter is too expensive. The mechanics said that the sensor is detecting that the converter is damaged and is influencing the computer. The car is consuming more fuel.
I have seen a little yellow stain on the spark plugs, people are saying, it might be the wires. But I haven't noticed any misfire.

Replacement of the cat with an aftermarket unit is most likely the issue here.