If it was me, i would just do a usdm WRX ecu/harness running the ver6 longblock. If looking for more strength you should up the rods and pistons. If you don't want to pay a premium for the JDM engine, a built USDM WRX 2L with gutted intake ports will work the same.
Ver5/6 are big ports whether WRX or STi. Impreza GT in Europe with an EJ205 engine runs the same big port heads. It's not STi exclusive