Tell me guys... I am confused because the STis in Australia/NZ are 2.0L while over in USA it is 2.5L. Why do they come in different models? Then what about in Japan ?

Yes it's 2.5L in the U.S., 2.0L in Japan. Why is that?? I don't know... Americans like more displacement, more torque, more power, yada yada yada.

For the reasone above and something about the WRC limiting the manufacturer entry to a production car with no more than 2.0L of displacement.

Most of Europe has large taxes on vehicles with over 2.0L displacement in addition to the WRC regulations. Neither of these is particularly popular in the USA, so the larger displacement than the standard WRX is a bigger selling point with American consumers.