Here's some info I found online while helping a friend build a hybrid motor. I will update this as I add to it...
In North America Subaru introduced the 2006 WRX with a detuned STi 2.5ltr motor, smaller turbo and and bigger brakes 4/2 piston to accommodate the power increase.
Also the 05+ Forester XT and 05+ Legacy 2.5GT also used the EJ255.
EJ255 & EJ257 blocks are IDENTICAL!!!
---> That does not mean their internals are. the 257 has stronger internals.
---> Both are semi-closed deck.
---> The difference between the two are the piston dish and heads. The combination of the small chamber EJ255 head and shorter piston dome make the 8.4:1 CR. The EJ257 has a deeper head and taller dome, making 8.2:1.
-Chuck