if it started right after your LT and mid-pipe i'd stick with things that are related directly to those
He has a picture of a car on his profile and it is stock.what kinda plenum do you have?
Man, what are the odds of that happening? When I first started reading, I was like "Well, what do expect with no air filter", which is what I assumed since a fly got in there until I kept reading......Had similar issues as you described but ended up being a fly in my Mass Air Flow sensor. Getting lean codes. Checked vacuum leaks, EGR. Ignored O2 sensor since I've heard it rarely causes problems but gets changed out. Ended up pulling MAF sensor and, with magnification, found a tiny fly stuck on the wires inside the MAF. Must have gotten on the clean side of the filter when I checked 2 days prior. Lightly blew out (in reverse) and out came the fly. Reinstalled MAF and all seems well now.