Oooh, more SCC disinformation.
I don't believe any of the major components are common with the 2.5 RS or Legacy (naturally aspirated) motors.
The STi 2.5 has unique: block, rods, crank, pistons, cams.
The heads are the AVCS 16v DOHC heads. The RS/Legacy etc use 16 v SOHC heads. Big difference.