HSV are currently using donor vehicles worth $40,000. The business case for importing high performance US vehicles just doesn't stack up because of the cost to land them here. A quick look at new Camaro prices in Aus shows a base price of about $90,000. Now HSV could probably get them cheaper but lets be realistic here, even if they could land/compliance them for $70,000 per unit, they're not gonna sell them for a dime under $100,000.
The fact that in its 20+ year history HSV have never done a single limited run US sourced V8 such as a Corvette suggests it's just not a viable proposition.