The perfect situation for this kind of swap is this:
Say you bought a blue ridge pearl 2002 2.5RS 4 door in November of 2000.
Say you used it as your daily driver and then in 2007, you decided to buy an STi.
Now, let's say that a small NASA satellite fell out of the sky, landing on your STi's roof, crushing it in, but leaving no electrical or driveline dammage and NASA paid you the full retail value of the car and let you keep it.
Now...you can swap that entire driveline....engine/tranny/rear/axles/hubs/brakes/wiring harness into your GC8. You'll have to spend a few months time doing this but when you're done, you have a very nicely upgraded Impreza with a nice, stock 07 STi power system.
If that doesn't happen, it's expensive.