I have no idea what Mike is saying Jordan "missed".

Having a really large exhaust on a N/A engine (with no other mods) is going to result in a decrease in low-end power and, in the case of this motor, minimal top-end gains. I don't know whether a 2.5" exhaust (with an aftermarket header) is going to be overkill or not, though - it sounds big. If I were interested in modding my 2.0 N/A, I'd go with a 2.25" exhaust. But, then, what's the point in that?

With that said, as I mentioned on the Tampa DSM board, you should swap another o2 sensor just to rule that out as the culprit.