Depends on which year RS you are talking about. But regardless, aside from either being SOHC or DOHC engines (sorry, forgot which year they switched in the Impreza) the general consensus is that, yes MOST performance parts will work for either platform, differences being mainly the exhaust and intakes, seeing as the cars are of different sizes and therefore have slightly different routing. Internally though, for like the cams and the like, they should be identical. Might also want to try a search in the Natural aspirated forum and see if there are any known conflicts with the particular performance part you are wanting to get.

 

I've heard that there are problems with using an ECU designed for the RS in a Legacy, not positive though.