Well, my muffler is probably a huge restriction (04-05 STi axleback), but the entire rest of my exhaust is wide open. I'm going to post back to back dyno charts with my exhaust cutout closed vs. open. I would like to see the same done on a completely stock car vs. aftermarket catback. I'm honestly surprised no one has done this.
I'm not saying there will be no change, but I'm willing to bet the difference is negligible. Then compare that difference vs. aftermarket downpipe. That will show you where the greatest restriction is in the exhaust system.