I know exactly what the difference is when I came from my AMD chipped S4 to my STILL standard RS4 - 3 years later! (well as good as standard)
I had a trial REVO remap. Supposed to be something like 420ftlbs 450bhp (not sure about that) anyway - the car was a fecking ANIMAL.... what a difference. So compare apples with apples.
Besides it is not just about the extra 115bhp over the stock S4 its the whole package. Lets be honest, I don't know an S4 owner that hasn't tried in someway to copy something from the RS4, I know I did. I wanted it all. It was starting to rack up to thousands and it would never be an RS4. Worse than that it wasn't worth anymore