Do you mean removing the brake bias valve to allow more braking to the rear?
If so it is normally done on classics that are running much bigger front brakes than standard and standard single pot rears to even the braking back up.
the blob wrx is already running twin pots at the rear that are a better match to a bigger setup then the standard front 4 pots so adding front Brembos/AP's
is a fit and forget in my opinion, no need for a bias mod.
If you over brake the rears they will lock up and send you into a spin under heavy braking