would it affect the performance in a negative way if i cut off the cat and replaced it with a straight pipe? I was told it would screw up the computer and the car would "shutter" when i got on the gas.

posting in the right section will help you get a quicker response, the car will run lean because the rear O2 sensor will think that the exhaust is rich and pull fuel from the engine, so you need a O2 emulator, and there could also be legal issues if your state requires you to have one

Moved to correct area.

Other then being illegal for street use? No, no real improvement to speak of other then being able to run leaded gas. You will also need to reprogram the PCM.