Maybe a bit of clarification is in order, or maybe I'm just a dumb shmuck.
It's been my understanding that the Legacy H6 rear brake upgrade entailed nothing more than swapping in new caliper brackets and the rotors (perhaps the dust shield stuff too, I'm not sure about that).
My basic point being that I wasn't aware new calipers were needed. Similarly to how the USDM GC 2.5RS' front calipers are the same as the USDM GD WRXs' are, only the WRX adopts new, bigger brackets to accomodate ~1" larger rotors, I have always been told that the rear calipers are the same among the three models, as well (GC Impreza, GD Impreza and Legacy H6).
Of course I could be wrong, or maybe people have just been upgrading the rotors and brackets and CALLING it an "H6" upgrade....