Thanks jamesb and markislive78 that seems to help a little

 

I contacted a well known fbody shop about this before I put my corsa on and they told me they had a 700 rwhp fbody with a single 3" exhaust and went to a single 4" muflex exhaust and on the dyno did not see a noticable difference in hp. They told with my application being under 500 rwhp a 3" single would be fine and not to spend the extra money on anything bigger.