The question you need to be asking is will Audi think either the C63 or F30 M3 will hurt the sales of the RS4/5 if it retains the 4.2FSI engine?
If that answer is YES then I wouldn't rule out the switch of engine, but you also need to remember that both these cars are limited production runs, even compared to the likes of the M3 and chances are they will still be able to sell every single one of them.