21PSI on the 38 is just at the limit of that turbo. You're probably already a bit out of the efficiency range.
I would personally recommend around an 18G spec turbo. That should give you a nice balance of response and topend without pushing the stock block too hard.
I would also recommend around a 750CC Injector Upgrade. But that really depends on your tune, current IDC and the nozzle you're running on the meth kit. But 750CC will give you a good amount of headroom.
As for brand, You can check out Blouch, FP, Tomioka, Kinugawa.
I would personally recommend the FP HTA71. FP has a sale on right now and the price is very good. It has great response and excellent topend. I'm not sure if its a bolt on application for the legacy though. If not, the other manufacturers would have something for you.
http://www.forcedperformance.net/mercha ... =WRX-Turbo