are you positive its the actual compressor? The A/C belt tensioner tends to go bad and can make a ton of noise just like what you describe, and its pretty hard to pinpoint the sound due to the rad being in the way.
Theres not much that can be done to fix the compressors on newer models, easier/cheaper to just pickup a new/used one. Not sure if the impreza ones are the same, they should be similar on the 2.5L wrx and possibly sti (it would make sense if they used the same one, but I know the power steering pumps are different, so who knows)
04xt/4eat, 16g, STI bits and bobs