EJ 205 - WRX 2L
EJ 207 - STI 2L
EJ 255 - WRX 2.5L
EJ 257 - STI 2.5L
according to your avatar, you have an 06/07 STI, so your car already has a 2.5L EJ257.
you'll also find info on the "open deck" or "semi closed" etc, here is what they are referring to, the amount of support for the bore in the block.
open deck
semi closed deck
closed deck
