Fortunately for me, the worst "carnage" my car has seen is when the paint was stripped, and the door handles shaved, when it was repainted Dark Cherry. Spent all its life since warehouse kept.
(I could really use a set of bumper fillers, but this is the best I've got !!)
I have an LS, with a 2 1/2" downpipe and stock exhaust. The downpipe alone (eliminating the "U" bend) works great. With a GS, I would probably go 3" with 3" exhaust.
Another must have mod is the "F" body 12" front brakes. I've had them on all 3 of my Regals (GS & LS). They are a great match to...
My wheel guys (Boze Forged) didn't deal with offset, just backspacing. There are calculators you can use to plug in wheel size and backspace to get offset.