I just found out that the AWD Rally 3 Fiestas definitely use a VENTED AP Racing rear rotor under their Alcon calipers, just like the fronts, and basically the same size rear rotor (285mm) for both gravel and tarmac, but a larger 335mm front for the tarmac events.
I guess they feel that the greater speed (even though the power is just a bit more than the Rally 4 rides, with yes, more weight due to the whole AWD conversion kit) (and weight) carried by the AWD class car over the R2T/Rally 4 FWD only cars warrants/justifies the greater 'consumables' costs of the ventilated rotors (not that that means anything/much on a $115,000 initial co$t rally ride anyway)