FAQ states that 5 door power output was about 10-15% down on 4 door models, until recently. How was this achieved, and can it be overcome?

 

This only refers to the 5 door STi models available in Japan (and some other far east markets). We believe power has been reduced to permit the Automatic transmission available on the 5 door to cope.

I would imagine that all the modifications are made by using a different ECU. Changing the ECU might not work, as the auto gearbox probably has different sensors. Probably your best option is to talk to one of the Japanese tuning specialists.