[thstone]

The O2 sensors provide feedback to the ECU which continually adjusts the mixture to balance performance and emissions. If you don't have the O2's hooked up, then the ECU runs in "open loop" mode with the stock mixture maps and can't adjust the mixture for performance or emissions.

In my Spec Boxster, I have the Fabspeed cat delete pipes which have a mount for the O2 sensors but still delete the cat's.