I'm thinking if the cat has a small opening to our dp then there really will be do performance gains in going to a bigger dp.

Unlike other engines where the dp is before the cat, then increasing the size of the dp will really free up the exhaust. Does this make sense to anyone?

the cat seems like the most restrictive part of the exhaust, but opening it up right after the cat is prob still good for a few hp, and maybe earlier spool up by a few RPM.
but this is one of the main reasons i went witha testpipe after my cat broke. i think a test pipe with stock exhauts would be worth more hp than exhaust with a stock cat, i may be wrong, but thats my opinion.
I got my test pipe before i got my borla and i think the test pipe was more of a noticeable gain then when i added the borla later. but the test pipe with stock exhaust sounds kinda weird, but good at idle, WOT was weird though.

Parts in order for K03 and K04 are
Manifold
Turbo
CAT
Downpipe


Even the stage 3 that came with your white car when you bought it from APR would have had the cat or race pipe after the turbo and before the downpipe.

And good luck to him too.

Although to be honest I pretty much know squat about a4's too, Still learning.

Why would anyone stick the cat b4 the DP?
Doesnt make sense. I know absolutely nothing about the stock workings of this car nor do I wish to.
Rod

apparently.

The cat also works best right after the turbo because it will heat up faster with the higher temps in the OEM spot vs further down stream. People move the cat to below the car to lower the temps under the hood or if they just dont have room behind the turbo setup they are using.

You have all that done to your car... and you didn't know that the CAT came BEFORE the dp???

well