This is what i hate about the ATO and government. If they owe you money, they make you wait, can't do a damn thing. If you were owed 2k in a return, and you didn't claim it, they are more than happy to sit on it for ever. But if YOU owe THEM money, god forbid! They'll call collections on your ass and hound you till you pay. Jerks.
Are you going to install the exhaust your self? If so, i'd be interested in taking it to the place thats going to tune it and ask them to get a base run first before mods, just for historical purposes. My base run in my caprice was with my headers and exhaust on already, so not very accurate of what stock is