i dont have a Gn (i wish i did) but i have a buick 3800 turbo i just went from a 3" dump into single 2.5" changed to a 3" full system same issues seemed to feel slower didnt have the same pull to it like i lost heaps of torque but was more the note change i think showed more torque & HP on the gtech and pulled 0-60 0.6 seconds faster on the gtech went much better again when i added extra fuel .in the end after quite a bit of tweaking the tune its way better than where it was before the exhaust change :biggrin: i was in the same boat was about to rip it off and fit the 2.5" back on.