1. How much power will I get from it?
Not enough to be worth the effort... unless you've installed a turbocharger / supercharger.
2. Will the sound be horrible?
Depends on what your definition of "horrible" is, but your exhaust will be much louder
3. How would I do it? Looking underneath, the last bolting connection I see is just before the converter. I guess just take it off, poke the cat with a steel rod, and shake out the debris? Is that right?
Good luck, that honeycomb material is pretty tough. I've heard that the best way is to pour gasoline into the honeycomb and burn it to a crisp, then hack it all out with a prybar & hammer. Be sure to reinstall the cat with new gaskets.
4. Will is set off my CEL or mess up the oxygen sensors?
It will probably set off a CEL for "ineffective catalitic" or something to that nature... but it shouldnt mess with your air/fuel ratio or "mess up" your oxygen sensors.
Rather than ruining a decent OEM catalitic converter, I would suggest that you make a "test-pipe"... basically a piece of pipe to replace the cat with. Cut two matching flanges, and the proper length & size pipe, weld it all together, get yourself some new gaskets, and simply replace the cat with the test pipe. When inspection time comes around, bolt up the cat, inspect, pass, then reinstall test pipe.
If you send me your cat as a template, or some really accurate blueprints, I'll make one for $100 (shipping included in lower 48 states). FOR RACING USE ONLY! :cough: :cough: :whistles: