I'll jump in cheekily in although MJ is the one to ask about this.
I looked into it briefly but it seems new headers would have to be one of the components upgraded to come near to 400.
JHM do headers for the B6 S4 which could be used for the D2 I guess. Looking at what they did to their S4 to get it close to 400 you would need a whole new exhaust system, sport cats or no cats, port polish, some kind of port spacer which they claim increases hp, and a really good remap. All of this would cost thousands, for a small hp gain its not really worth it especially since those crazy S4 B5 V6s can put out so much power and torque with good turbos.
Well designed headers are really expensive, and you will spend at least another £2000 on a well designed exhaust and sport cats.
Fun video , I like the music
https://www.youtube.com/watch?v=3n7xVwjWWD0
You could go here and spend probably over ten thousand on inconel headers an exhaust
Great place
https://www.primarydesigns.co.uk/exhaust-design