I am not Type rated on either.
As per the type rating goes, its not necessary to get a FAA Conversion done. Same for us. We have an ICAO license and some of my friends head to France or USA for type rating on ATR etc. They get the training done and get a certificate from the TRTO on the basis of which the home country's CAA endorses the license with the TYPE rating. However, the TRTO has to be approved by the home country's CAA( in your case CASA).
Under PPL/IR you wont be able to fly commercially(incase in future someone comes up with a prefrence of FAA license as a prequisite-FAA CPL/IR would be better).
If you are headed to US for type rating and are getting a conversion done. Best thing would be get the CPL/IR/AMEL instead of a PPL/IR/AMEL. It might take a bit longer but certainly would be more beneficial.