How much restriction can a Cat be?

[rfkellner@snet.net (Rick Kellner)]

    I am in the process of replacing my entire exhaust with a TT system and
a TT down pipe so everything is off including the catalytic converter.  I
have never take this one off and was surprised to see how it was put
together.  It is just a can filled with screens!  I know people replace
these with high flow cats but how much restriction can these be?  Anyone
really notice a difference between the stock and an upgrade or even a
straight pipe?
Rick Kellner.
84 8v