RS5 bumper is not compatible with S5. Exhaust is different, size if silencer(rear) is different. Pretty much to adopt RS5 rear bumper and exhaust to S5 is mission impossible. BUT, if you are crazy enough with huge amount of work everythin is possible.
Just my question is why to do it? S5 is S5. Even with RS5 rear bumper it is still ONLY S5. Do not be like many BMW owners that put 335i sign on back of their 318i...