Its because the JDM engines will result in you having to modify some things just to get it to work correctly. Or find work arounds on it. If you want it to run like stock it'd require a new wiring harness, you need the sensors the EJ207 uses that the american engines do not....Acvs sensor being one of them, JDM ECU, and you can see where this is headed. Yet, with work arounds some people have the EJ207 with a lot less work. I couldn't tell you as I have not done that modification. If Basco ever returns to the site...he has been MIA lately. He'd be your best bet for information on that route. As he is running a EJ207 in his 02 wagon and making rediculous amounts of HP with it.
Other options are a hybrid route...using a EJ255 bottom end with your EJ205 heads. Which still requires some small amounts of modification to pull off. Psinuse is running a 2.34 L hybrid that is another option. Thirdly you could just buy a EJ205 engine and replace all the insides with forged parts and basically a have american low end EJ207 if you do head work and cams as well. Last option is stay stock....
Hopefully some other people will chime in with their thoughts.
378awhp/373wtq Custom Dom 3 & CBRD tuned 2006 STI.
"When in doubt...flat out" - Colin McRae