Dare I ask what you weigh?
I found a cheapo way (free) to get your car weighed accurately is at the tip . weighbridges are spot on accurate.
Design regulations used to be state by state, I have had four cars engineered in NSW,used to be strict, but fair. Queensland,where I live now, at the same time was comedy central, you could get a Morris minor registered with a 7 lire Chev V8 in it. a few years back the rules were modified so a car certified in any state could be registed in another without re-doing the certification. AS LONG AS THE CAR IS STILL REGISTERED- don't ever let the registration lapse.
This is the link to engine mods these days, The engineer I was using in NSW, told me they can no longer use discretion to go outside the rules.https://www.infrastructure.gov.au/vehic ... 011_v3.pdf
and that's just for engines!
The complete list for every other nut and bolt you might want to change is here.https://www.infrastructure.gov.au/vehic ... _ncop.aspx
I think the basic idea is to make it near impossible to even think about modifying you car