Even setups with the 02 sensor pre catalytic require having a cat on them to run properly/effecient. After doing some heavy math on exhaust flow on a 2" system, straight pipes (Header/No cat/Muffler) flow at 407cfm while the same setup with a Magnaflow cat (and other brands apply to being high flow Ie "Borla/Dynomax" so on and so forth) They were running Header/Cat/Muffler and it did 397cfm.
Long story short, Cat's improve gas mpg while keeping the vehicle effecient and exhaust clean(er), plus help Gain power in the powerband. A friend of mine has an 97 plymouth with no cat, and lemme tell you, Not only does OBDII systems not know how to recalibrate for that item missing but it sounds like an angry bee coming up behind you spewing gas droplets out the back. Yum Yum.
For $50 to be legal. Beh, spend it <img src="/forums/images/graemlins/lol.gif" alt="" />