I've seen similar cars to the one above at the Audi Forum in Ingolstadt. I believe these are RS 5 Cabriolet production mules as the ones I saw in person also had RS 5 instrument clusters as well. This would lead me to believe that the RS 5 chin does in fact fit a non-flared A5/S5. I doubt they would have produced a special bumper for the mules (unless RS 5 Cabriolet will have no flares... which I further doubt). Likely they wanted to perfectly simulate cooling so they fitted the factory RS 5 chin... typical on test mules. My guess is you could do the same.
Another option might be the new body kit from Rieger. This appears to be a replica of the RS 5 setup. Here's a link.