They just space the O2 sensor back so that there's less exhaust going to them. Sensor one says, "We have 1x of flow!" And sensor two says, "We have .75x of flow!" And the ECU says, "Good!"

 

ah ok, well that works. Will this throw a code a lot? or just sometimes or what.

 

ecode must have done a few things to alter those results. someone with a k04 recently dyno'd their a4 pre and post installation and saw a 0hp peak gain. the powerband looks better in some spots, but that was it. going from stock exhaust to tp and full 2.5" exhaust will net you about 20 at the crank if your chipped. but not to the wheels