Do the 2 hole, performance filter, catback exhaust. If you're planning to do something to the engine itself, make sure you don't try to hide it. Cops will be alot tougher if you try to hide mods.
Also, its not a bad idea to look into suspension and brake upgrades, they're perfectly legal and make all the difference. A tight ride will be rewarding when you take corners. I don't know what you're willing to do in weight reduction, but it will give more power to weight and improve cornering. The commodore V6 is actually a very powerful engine when considdering others, the car is just too heavy to let it go.
An ECU upgrade is a good thing to look into, just not sure if it's counted as an engine upgrade. Better check on that one.
A good set of tyres will make the car feel more responsive and stick to the road better, but if you want to let the rear end go, use a hard compound.
The best mod I've done so far is the red engine covers, it makes the engine comp look sweet and obviously adds about 5 horsepower =)
It all really depends how much money you really want to spend, even with the laws in place, you can still knock at least a second off your 0-100 time.
Don't mind those knobs telling you to look elsewhere, coz I can't work this bloody forum out either.