I'm not 100% aware of the rules but just from a quick scan of the Vicroads website it says that 'if your car is modified, you may also need to obtain an engineer's certificate and do your own calculations.'
So from that if you started to mod your car you'd need to work out the new P/W ratio and if it did exceed the figure then it would be illegal to drive.
By the way, welcome to SAU. If you have a question, you just need to post it once in the relevant area, not multiple times. That way all the answers will be in one place..