Honestly I don't think cost has anything to do with it. You can't compare the cost of a car in the US to one from overseas. Cars cost more or less in different economic environments due to a number of reasons. In Japan the STI costs less than the US STI. You could buy one and have it shipped over to the US for under $35k. If the cost of the Aus STI is $40k USD, that's too bad, your car prices aren't very good.
If you look on Subaru of Japan's page for the 2003 V8 Twin-Scroll STI, it does not use forged pistons. Here is a babelfish-translated segment from the engine technical description:
The engine of STi has made most the private design in order to guarantee the potential with hard travelling. For example, as for the piston the casting aluminum alloy make which quite guarantees high strength. Also the crankshaft administering Tufftride processing,
They also have a picture of the piston and rod on the page with the caption "High-Intensity Cast Piston" This applies to both the RA Spec-C and regular STI street model.
So someone should go tell SOJ their website is incorrect...
As an aside, you can get those forged pistons for much cheaper than $1022, they are about $500 for a set...
There are no international/JDM spec motors this year. There are JDM spec engines, and everywhere else in the world has the same engine from last year. They are completely different.