Mazdatrix told me so.
They stated every new S4 rear iron comes with the updated casting seen on 13REW's.
They also recommended I replace my rear rotor housing becasue they state every rear iron break they have seen causes the rear rotor housing dowel hole to "oval". I asked for some measurement tolerences from them since the FSM doesn't specify.
I am awaiting a response.
Since I have a hair line crack and I pushed 17 psi once on low compression rotors when my WG went tits up(15.4 psi is my normal boost), I am skeptical that my rotor housing is "stretched". I guess I will see once I pull it apart.