Can you post up a picture (or link) to the Milltek or Supersprint dump pipes you have found?
Im sure that you could get an exhaust shop to weld a reducer or a flange on to your stock cat back and also onto the downpipe to mate them up.
Otherwise, perhaps look at getting a custom dump pipe made up for your car. You are in Sydney, so there are plenty of choices for custom exhaust shops down there.
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