I recently had my rear bumper off to make the cuts for a Gt500 valance. On the inside of the bumper, Ford has already molded a trace pattern for the cuts/slots of Boss/Gt500 valance. I would wager Roush has used the same pattern to fit their valance. It is probably just a matter of removing the Roush piece and installing the 500 valance..
here is snippet from American Muscle on product application of the Roush Valance
Application. This Roush Rear Valance is designed for use on all 2013-2014 Mustang's including the V6, GT, BOSS and Shelby GT500 models with a Roush catback exhaust system installed.