I failed to mention the bad tune had me running super lean which fried a cat. My car acted similar to yours running lean but I don't see where you're getting lean codes.
Did your mechanic come up with anything?
Here's the info on the H pipe if you consider going this route in the future.
The link wouldn't paste so just google the part # and discription and the Carid.com website will come first I believe
08 GT 5spd
Spectra CAI, Comp Cam Detroit Rockers, Kooks headers w/catted H-pipe, Revolution Automotive dyno tuned, FB430 race mufflers, 3.55s, JLT catch can, BMR Lca's and relo brackets. Prothane motor mounts and sway bar bushings.
20% tint, smoked markers and tail lights w/sequentials. Clear front signal covers, series 3 chin spoiler.