It has to be 16mm or 17mm. Wrx I know are 17. Its not going to be standard. If it is beyond tight that a 1/2 impact(even cheap one) can't get it off your pan threads should be fubar.
Two options, 1st being easiest
1) got buy extractor sockets tap the size that fits on and get ratchet/breaker bar and be done.
2)Get a hammer and a chisel(if you have access to an air hammer with the right bit it will be easier) make a small cut into side of plug head(don't hit pan),so you have something to bite. Then angle chisel to where it will spin plug left to break loose.