I don't want to single out all Borla owners. But I remember the guys in the G8 forum did not like the Borla installs on their cars. Terrible droning was always the comment. Magnaflow was a good alternative. Corsa was the best, but most costly.
FWIW, the G8 GXP and Denali engines share a lot of similarities.
Interesting info on the G8. I feel the body of the Borla muffler was just too small compared to the Magnaflow to suppress a lot of the resonance. I'd love a Corsa muffler but I don't plan on racing anything so I can't justify the cost. I had just a resonator delete on my 09 Denali and that sounded good enough for me. The 12909 swap is pretty close to that.
Did you get tuned yet?