That Impreza is deffo worth more in bits given the tuning parts on it, plus at this age theres plenty of folk willing to buy second hand used parts to replace broken parts on their cars or upgrade a WRX. STI 6 speed gearbox will fetch decent money, so will the engine, wheels, brakes, seats and interior, both exhausts that he has for it...etc etc. The bottom really seemed to fall out of the fast Subaru market a while back.
Its a real buyers market, you could try selling it for ages and get what, £6-7k now? Where as the gearbox is a grand plus, the bare engine a grand/grand and half, brake calipers maybe 400-600 all round, coilovers on it will fetch 400 maybe, wing and boot for 350-400 plus, seats/interior 600 odd...etc etc.
If you know a friendly garage or have the time and inclination to break it for parts yourself, will probably come out better although you won't shift everything quickly, especially at this time of year.