There's always alot more space than you'd think behind door cards. Just cut a section of polystyrene out of behind the door card in the old legacy and jammed it in there, worked like a dream, imagine impreza's follow a pretty similar trend. And as much as most people say can the rear speakers i still prefer having a strong backfill.
Got the polk DB's bigger siblings in the current car, front and back, and those things are absolute beasts.
Without wanting to chuck large wads of money at it find something with 4x50-60w. Nothing wrong with going for a matching pioneer one.
And for sub i'd say a 12inch with atleast 400w rms. With the old preboxed 8inch 250w i had i found the rear seats really limited what could be heard from it and needed to be cranked obnoxiously loud to hear the full range.
Also impedances is key with subs, don't go rushing in without really researching what each sub needs and what amps can provide that at the required impedance.