once you remove the cat and your ''check engine'' light comes on it puts your trucks computer into safe mode, which makes your truck lose performance because your computer is trying not to blow your truck up. once the cat is off and you weld in or clamp on your straight tubing to fill in the gap, youll need to tap that tubing and put a bung in there for the o2 sensor which im sure your aware of but, that engine light will remain on.
at this point you can do one of two things. find a little electronic piece (cant remember what its called) but you remove the o2 sensor and plug in this small electronic that mimics that your o2 sensor is running how it should and itll take the engine light off the dash. my buddy bought one for about $30 for his OBS. my favorite solution is the spark plug non-fouler. put the non fouler into the exaust then screw your o2 sensor into that and get your computer codes reset (autozone can do it for free).
problem solved!! a lot of info but i went through this headache for a week and it can get under your skin pretty easy