Your post is confusing. Do you have the stock RS, brullen or stromung mid pipe?
I would go with 2.25" piping. Keep the low end torque you got.
The cat has a flange on it and so does the mid pipe. There are 2 spring bolts connecting the 2 and the donut thing is a gasket. You can replace this if you want but most likely you can resuse it.
You will be able to bolt the header/cat setup to your stock mid pipe if the flanges match up. only way to find out is to crawl under your car and find out.