g'day mate, I have a AU2, with a full system. I have stainless mandrel bent from the cat back with mild extractors (stainless extractors can create to much under bonnet heat).
I would definately recommend going for extractors as soon as money permits!!
I only have one muffler on my system, its a tiny genie turbo muffler, and holy *@%$ does it make some noise. It sounds awesome with some acceleration, but I think it drones a little at freeway driving. I am about to stick a resonator in the middle, to cut down on this drone and bring the noise to a level which is less likely for the neighbours to complain :-) (cant drive it before 9 or after 10 lol) But yeah, trust me, if you are going to go for a single after axel muffler, and its only small, it WILL be LOUD!
AU2 S-pack Owner: Silhouette Black, XR6 Wheels, Tickford Sunroof, C2R rear bar, Checker Plate Floor Mats, Chrome Door Sills, Blue Backlite Dash, Blue Painted Brake Callipers, Advanced Extractors, Magnaflow High Flow Cat, 2.5" Stainless Steel Mandrel Bent Custom Exhaust, Genie Muffler. Sony Explode 50w x 4 deck, Kenwood 140w speakers (front and rear), Boss 1200 watt Amplifier, 12" Orion Sub in custom box. More To Come!!