I drive a 99 civic si, with a jackson racing supercharger, dc 4-1, "hi flow cat or test pipe, whatever" and a full dc cat back exhaust. However the cat is new and it didnt have a spot for the stock 02 sensor, so now i get a check engine light, but besides that the gas mileage seems to really suck. So i guess im wondering if it it is bad to have the car set up like this, limp mode etc...and just letting people know that the hi flow cats on ebay have no 02 sensor spot. The stock cat is going back on soon, it also made it really loud, and smelly.

Hi-Flow Cats imo

 

Sounds more like what you have is a test pipe.

oh, and by the way, an o2 sensor cel won't put you into a limp mode. I got 27-30 mpg on a b16a2 from a 00 Si in my old hx with NO oxygen sensors connected.