Ecutek dealers can retune JDM ecu's if they have the neccessary hardware and software. Most don't have this stuff, but it is possible.
IMO, after doing these swaps it has become apparant that most engines pulled from a wrecked car have minor broken bits. If it's a JDM engine, those bits may not be readily available here in the states. Most of the time, it's a two week wait for anything JDM.
With that said, the STi makes more power out of the box but the US aftermarket certainly isn't lacking in EJ205 parts. IMO, it's easier to use the USDM engines for a swap (EJ205 or EJ257) financially, logistically, and repair-wise down the road.