So to mirror another review here, yeah, this thing makes it loud with an aftermarket cat back exhaust. Loud, but it sounds so sick. CTS makes good stuff and I'm happy with this product. A couple things to note there though.
1. As it says in the description, this will throw a CEL. I have 2016 A3 Quattro. You will need to get an O2 spacer to fix this. I got mine from UROTuning, worked like a charm.
2. If you have access to a lift, use it. I had to do this on jack stands and it is not high enough to get the stock downpipe out in one piece. I ended up cutting it to take out in 2 pieces. Be patient, as the instructions say, it will come out just need to be easy with it.
3. This is important. If you have an aftermarket cat back already (mine is a Borla S-Type), this downpipe will NOT be long enough to connect to the cat back. I find it odd that there is no mention of this. I ended up having to go to NAPA to get a custom length of 3" pipe cut and a 3" exhaust pipe sleeve to get it all to work. Not the end of the world but I wish I knew this up front.

All in the A3 sounds bad ass now. It turns heads for sure.

I received the order well packaged and the Downpipe looks well made and top quality. I made the mistake of not thoroughly researching the sound this thing made with a non-oem 3" Catback from Borla. My Catback already made a nice sound with some crackles. But after I put this DP on, it was like Bruce Banner turning into Hulk. It's ridiculously loud to the point I am embarrassed to drive my beloved A3 Quattro. My car vibrates so much more and I need to drive so slow to avoid accelerating in my neighborhood at night to avoid waking everyone. I am going to try and put an Ultra Quiet resonator. If that doesn't work I will have to take the CTS DP off and put back the OEM and try and sell the CTS. It literally gives me a headache from the unbearable drone this DP creates. I think this would go well with the OEM Catback.

Great product, fast shipping, awesome sound. Fitment was perfect.

Fit perfectly, no leaks no CEL and the turbo spools up way faster. The power and lower RPM has increased as well! Great product!!

I purchased the CTS Catted Downpipe for my 16 S3 and the manufacturing is top notch. The factory DP is a little tricky getting out but the CTS DP slides right in and bolts up nicely with no alignment issues! I was even more impressed with the sound...I have the factory catback and resonator but with the CTS DP I couldn't be more happy with the sound of the car now! The DP along with the Stage 2 Unitronic ECU/TCU tune is an awesome combination. Great product and I highly recommend!