a 20g is worth more than high 300WHP (with proper tuning of course). Any of the IHI turbos can get into the mid to high 200 WHP range. IMO like /\ said, the 20g is just too big a turbo for a stock rex in us fashion.
IHI turbos do NOT like high boost at high RPM's, they like to be tapered from 6K on. They will let go if pushed to high boost/high boost at high RPM. IHI turbos aren't spectacular turbos, they're middle of the road turbos for middle of the road duckets. Keep in mind subaru chose this turbo as OEM for many of it's cars, so it's by no means cheap, but by no means the best you can buy either.
And please do your homework on Tuning and EMS's. Tuning is a VERY important part of keeping these cars alive. Detonation kills turbo charged cars in short order. There are now tools out to tap the stock ECU and use it's power for the benefit of mankind. I would suggest looking into ECUtek and CObb Protuning. Custom tuning is needed, just say no to generic maps. The stock ECU is both very powerful, and very forgiving (meaning it will let you mess up without going BOOM, within reason of course). HTH.