Retune WRX to 300whp? I don't think so. It would require 430hp at the crank.
BRZ has more power to weight than the WRX, and by retuning you lose engine warranty. With current 2.5L - I would not want to own it w/o warranty. I might consider a new WRX with 2.0 or 1.6 engine, if they still have a hatch.
And
there's already a long thread on this.