Driving hard with a clogged cat is a good way to blow up your engine. Not sarcastic. The cat (catalytic converter) occurs right before the rear O2 sensor, and just after the front O2 sensor in your exhaust. If it is clogged, then exhaust gasses can not pass easily, which will increase backpressure, and increase knock to an unsafe level where you'll chip a piston or something else uncool.
That being said, you're safe for a little until you replace it, since it may not be completely clogged - or particularly clogged at all. The P0420 code just indicates a drop in cat efficiency. A test I've seen used to test for a clogged cat is to drive around with a warm engine for a bit in the dark, park, and look at the cat to see if it's glowing red. You may also smell sulfur/rotten eggs from the exhaust.
From what I've seen, the life of a cat is only about 150,000 miles anyway. Buying aftermarket means you're paying $50-75 instead of a dealership's $600.
