i went through this in nsw this time last year, my car had full exhaust (no cat) bigger rims an extensive modifications in the engine bay, i took it to a place to get a blue slip, luckily i had a engineers cert to go with it, although it only covered basic things and exhaust and rims nothing in engine bay. i am not sure how it works over there but it is only randomly chosen and not very often mind you that you you need a full inspection, i got cleared with a blue slip and then organised all my rego only prob i had was stupid woman at rta put the vin chassis no wrong and sped 3 days trying to find matching codes for import papers, wasnt impressed.
so as long as you make the effort to try and make it look streetable/sound streetable with everything where it is suppose to be you should be ok, but as i said done in nsw not sure if it is compulsary for you in wa