I like to gut them because they always get clogged! If you do a simulator box is needed to keep the ECU happy. A short search on Google will get you what you need. Avionics guys here at work have made them for years.
the circuit that mimics a perfect o2 sensor is a must when removing the cat.
but you would actually lose hp if you just gut it.
you would have to weld in a piece of pipe that has the same diameter as the mid pipe.