I think you’re right about the similarity between the brake setups between US to AUS. The 05/06 GTO was fitted with an LS2 the same as the VZ HSVs which would warrant the slightly larger brakes.
I’d look into some braided lines though if you were to try and source some VY lines. Even if you aren’t planning on pushing it hard, nothing better than a little overkill. lol. Especially when fitting those massive CTSV brakes I seen that you got in your build thread.
I’m actually pretty keen to see how it turns out as I’ve been looking at the kit for a while but can’t seem to commit to getting them for some reason. Maybe due to the fact that wheel choice would be limited due to calliper clearance.