I have been asking the same or similar questions all over the internet, even subaru of america has been queried.
What I have found, is either the info isnt out there, or apparently ppl dont know.
After asking several question across forum both in the UK, and the states, I have finally found out at least for my example and 05 2.0 turbo out of a wagon, being mated to the electrical system of an 03 WRX that had the same motor, (then going into a VW) its a direct connection, nothing is different. From what Im reading and being told whats different from 02 to 05 is the cars electrical system. When you get into the STI's you have things like AVCS, etc. The 05 cars had the immobilzer tech in the car itself, but not the motor.
So my guess based upon what Ive learned,...would be, it would be fairly straight forward "if" the motor is the same 2.5, to a 2.5, or a 2.0 to a 2.0. Its the car itself, that has all the differences. At least for the people that do have answer, that is what Im hearing. Minor changes if any, a sensor, or a manifold perhaps, but nothing major.
Anyone am I wrong? Please correct me. The question has been asked on this forum ALOT, and they are simply single post threads with not so much as one aswer.