This article is from the VW Performance FAQ, by with numerous contributions by Jan Vandenbrande others.
A:There is a steel doughnut going right before the cat.
This stock doughnut has a two inch (approx) hole in the
center for the exhaust to flow thru. It can be replaced
with a doughnut with a 2 1/2" (approx) hole to make the
exhaust breathe a bit easier. Not a significant power
increase, mind you. [mgm@royko.Chicago.COM (Marty
Masters)]
 
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