I don't remember RS lines being nothing more or less than true competitors to M in the past or present. S cars were a bit in no-man's land so they were hard to market. As you said, 408ps might not be a true figure, only official, but would be good enough to put competitors in their place. I don't know why they chose 408 and not 420ps as new A8 will get, but everything has a marketing reason. I guess they don't want to stress advantage on engines anymore and want to prove superiority in chassis design. Anyone can build strong engines with latest FI technology, but only few have got a talent to iron out the chassis balance.
Best
D