i would personaly go with the test pipe. you get the power sooner due to the less restrictive flow, and if you guys are about the sound you do get a little more audiable noise. i have a test pipe on my wifes A4 and she loves it. now i say go with the TP if you dont mind getting a CEL, or taken it off to pass smog. now i have a cat on my A4 with a full custom 2.5" exshaust and Stage 1+ and i can feel that the turbo doesnt spool cuz of it.

i would be carfull about getting something off of ebay, things have a way of not fitting right.

good luck

 

I have that angled O2 mounting device but it is on the FRONT bung not the back one. Does that make any sense?

 

i believe that it is the second sensor that reads how efficient your cat is working. so your gunna want to put it on the second one