The only real difference is the US models AM radio is fixed to 10kHz incriments, whereas our AM band goes up in 9kHz incriments. Apparently FM is slightly off too, so you may get some sibilance.
The US Pioneer Premier models are the equivalent of the Australian models, the US basic Pioneer models have slightly less features compared to the Aussie version.
You do realise that you get no warranty though? If you have even the slightest problem, you will be charged for any repair that Pioneer Australia has to do. You'd want to hope you don't damage the RCA fuse while installing it or don't mess up any of the speaker wiring because your savings by importing will go down the drain.