The S8 has been spied again. Now, I don't speak Spanish, but a quick online translator says, "everything point to the S8 will using the RS5's 4.5L engine which will produce over 500HP in the S8." The doesn't seem credible at all since the RS5 uses a 4.2L motor.
Additionally, George has found out that Audi is working on a new turbocharged 4.0L V8 that will replace the 4.2L in the A8 and a more potent version of the 4.0T to replace the V10 in the current S8. That engine will presumably have about 450hp.
btw, George, the current S8 has 450hp, not 500.