Most N/A exhausts choke down to the stock diameter where they meet up with the cat. Does the RS*R do this? I know my Tanabe RM does, and so do the GReddy exhausts. Anyway, that's a pretty big restriction, so you might want to have that flange cut off, along with the restrictive piece(a couple inches behind the flange) and then have a new flange with a short section of 2.25 or 2.5" piping welded back on, and then mate that to your 2.5" Carsound and downpipe.