Assuming that you're talking about 2.0 litre engine (not the US 2.5 version).
Falcon WRX use EJ205 engine that has open-deck design and use oil splash to lubricate cylinder wall. This means that it is
for 300 - 350 h.p. if you modify it.
Falcon STI JDM use EJ207 engine that has semi-closed deck design with oil squirters to cool piston and lubricate cylinder wall, which can handle more h.p.. (take more boost than EJ205). There are lots of different between the two engine. STI being much superior with better (stronger) parts all around.
