Technically, if you reduce the boost the car will be P plate legal. This is fine according to vicroads... if you can prove it. You'd need to ring 13 11 71 and ask for the Road Safety dept. They will prolly tell you you'll need an engineers certificate (for the mods), then prolly a dyno reading at the engine, and a roadworthy. After all that they'll put a notation on the system so whenever you're pulled up, the coppers can check the mods and make sure they're still in place. If they're not big fine then canary. Give em a call to confirm.
Here's how its worked out:
A high powered vehicle is any vehicle with a power to weight ratio over 125 kW per tonne, or with a capacity to weight Ratio over 3.5 litres per tonne.
The weight of a stock standard vehicle is measured in tonnes (1000 kg = 1 tonne) capacity is measured in litres (1000ml = 1 litre), and power is measured in kilowatts (kW).
The power to weight ratio (kW divide by Tonnes) must be 125kW or under.
The capacity to weight ratio (Liters/Tonnes) must be 3.5 litres or under.