i dont know for certain, but I looked at my 09 sedan (after wondering if the bumpers could be swapped) and they end in front/behind the front/rear wheels, so I do not believe they will match up correctly.
My thought is that even if it does fit the track on 2011 and newer WRX's is wider so without additional work your wheels would look off. Correct me if I'm wrong.
Yup, I just found this online. Track increases of 1.3 inches in front and 1.5 inches in the rear for 2011 and newer WRX's. Seriously doubt a bumper will work on the old REX without decent modification.