As you know, the STi version 8 in all markets other than the USA is still the turbo 2.0L. In addition, the WRC class does not require homologation like the Group A cars did. A participating manufacturer need only build 50 examples of a car to meet the WRC class requirements.
The only possible issue is how this car could possibly qualify for SCCA ProRally/ClubRally. Perhaps limited import availability of 2.0L STi's for racing? I don't think there is any information on this yet.
As for the reason behind 2.0L vs 2.5L, it all has to do with emissions and gasoline. It is not currently possible to build a reliable turbo 2.0L producing 276+hp using USA gasoline and meeting USA emissions regulations. The UK has similar quality gasoline, and the 2.0L STi produces only 265 hp. The Japanese gasoline has a significantly higher octane rating. With higher octane, you can run more boost without knocking.