My cat is clogged with coolant and oil from a blown head gasket. I was going to just hollow it out, but I heard some crap about stagnant flow messing up your air/fuel ratio if you hollow out a cat. And because of how close my cat is to my o2 sensor(You take off the nuts marked red, pull that portion of the manifold off and the cat is right there) I was worried it might.
Anyone who has experience with this, feel free to give your .02 on the matter.