The exhaust is in 2 parts, the first joins the turbo and has the pre cat and cat in it. This is called the down pipe and runs from the turbo to just after the main cat.
The rest of the exhaust is called a cat back, because it runs from the cat to the back of the car, and includes the back box.
The main restriction is the narrow down pipe, and the pre cat and main cat installed in it. Significant gains can be had by replacing this particularly restrictive pipe. It will spool easier and flow much better. I believe (could be wrong) that the stock cat back isn't
that restrictive and would only start to get in the way at higher rpm, in higher states of tune. When it's capacity to flow easily is reached and it starts to generate significant back pressure.