There were a few differences. I had to splice the plug connectors from the stock fans onto the fans I already had on the radiator ... no big deal. The radiator hose on the bottom of the drivers side of the radiator was a little higher by a couple of inches. I used the longer rad hose from the other car and trimmed it a bit to make it work. It's not ideal, but it'll do for now. I may end up getting a Forester specific rad for it or just replacing it with a stock unit. Also, the rad brackets that hold it to the frame at the top are different and don't work. I've left them mounted to the frame even though they aren't attached to the rad in any way, just so I won't lose them, haha. Other than that I had a few air bubbles that had to work their way to the top, but everything seems to be functioning fine now!
Also, the new radiators Koyo makes are the thinner core. The one I had for the Impreza is a bit thicker.