Drivetrain losses on the Subaru AWD setup are unusual high. Most RWD cars have about 17% drivetrain losses. FWD cars maybe 15%. Even AWD DSMs are generally low 20s. Subaru's system is a full equal length drive to each wheel though so you're going to see ranges of 25-30% drivetrain losses mattering on the dyno. Thus, when you see a turbo speced, it's often in reference to another car's drivetrain losses which are lower.
The usual budget option is fuel pump, sti injectors, used VF39, used STi TMIC. If you're good, you can get all that for $1000-$1500 and with your exhaust (you'd need to add an up pipe as well), you can use the cobb map for that setup.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD