I leave mine off too, and it runs a lot better. I figure why use an O2 when the PFC reads the cells anyways. I guess if you aren't fully tuned to a perfect A/F ratio under light throttle, it cleans things up a tad.. But, yeah.. I have no emissions devices on the car anyhow, so WTF?
Disclaimer: Heh em! I mean, you should never remove the emissions devices from your car, it's illegal and bad for the enviroment.
This vehicle is used for off road and race purposes only, and is never driven on public roads.