as everyone has said EXHAUST! then i would work the inductions, get some sort of cold air intake (i.e. t-series) then a air filter (i.e K&N) if you get a good price on these, I would spend it on a cam, probably one from Jim Mock Motor Sport in Victoria!
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!!