The stock cat-back will bolt up with no problem but it will look like the downpipe is trying to eat the cat-back it is so much bigger in diameter.
At idle, it is a little louder. Not very noticable but we did have a stock car to compare it to. WOT opens up a bit compared to stock but I have not had anyone that has been in the car complain about the sound. It sounds good.
The main difference in sound is that you can now hear the turbo spool when you are in boost. I think it is because the stainless tubing does not have the same sound dampening qualities as the stock cast pieces. Personally, I bought a car with a turbo and do like hearing what I paid for!!!
I am having a cat-back built with a 6" round Magnaflow muffler next wednesday. It will be mandrel bent 3" and should be similar to the Turboxs exhaust. I just wanted the round canister muffler rather than the rectangular Magnaflow that Turboxs uses. I will comment on sound after it is installed.
The only other thing that I think I should comment on is that when you replace the stock downpipe, you are also replacing the donut gasket at the cat-back. I think this is typical with most aftermarket downpipes. Since you take away the flex joint and the downpipe is bolted to the motor, transmission, and car body, you do notice that there is less flex in the exhaust system but it is minimal. I have not actually had anyone comment on it, I just noticed it myself (I am pretty picky about sound, vibration, etc.)
In all, I think this is a perfect upgrade for the WRX and it is less expensive than most. Alot of bang for the buck horsepower!!