You can run the trans in L7 (if 8 speed) or L9 (if 10 speed). DFM won’t run and you can at least see if that’s indeed the issue. FYI, it never deactivates cylinders at idle no matter what, so what you hear on startup, in park, at a stoplight, etc...that’s all 8.
I have the GMPP exhaust that retains the stock tips but I think is otherwise the same as the Borla you have. Can’t say I share the same opinion about it, sounds great to me. Even with DFM active, it just has a slight lope to it but not the obnoxious helicopter sound the old AFM motors put out.