Hey Yissy,
I don't know if you are still having the exhaust issue, but I went through this for many months, trying several different types of systems on my SSV Wagon. The hammering inside the cabin was insane while afm was activated. I was hesitant to have it turned off at tuning, as I am getting 8.2 litres per 100 on regular interstate trips. If I drive in manual mode, the afm does not kick in, but the fuel consumption is around 10 Lt per 100k. What I have finished with is 1 3/4" Headers, High flow cats, 2.5" system (Hurricane) and the original Holden mufflers. It has a bit more grunt than stock, but it is still very quiet. I am not at all happy with the note, but I know of no solution. If you have found one, I would love to hear from you.