Subaru engine swap

the 2003 Impreza 2.5RS should have a EJ253 (2.5L EJ series), the Legcy is probably running a EJ251. the EJ253 uses a MAF sensor, and the EJ251 uses a MAP sensor. the EJ253 made improvements to the head gaskets because the EJ251's were failing. as well as engine management. so yes, the engine is a drop in replacement, and will fit the transmission. but you need to change more than the engine, because of the changes in management software, you will likely need the ECU, intake box, sensors, etc as well.