It seems like the new Porsche Panamera Turbo S was something that would keep the Aston Martin guys sleepless so they decided to make their Rapide more powerful. Thats why they invented their own S model, the Rapide S.
If the 470 hp in the “basic” Rapide is not enough, the expected 510 hp in the S version will help you out in that case. The S engine will come from the Aston Martin DBS, so its no new development. With more power there will also be an demand for better brakes and suspension and thats what the Aston testers are working with at the Nurburgring right now.
As you might have noticed the beautiful Rapide body will get some smaller changes, like a lower front bumper with bigger air intakes and a slightly bigger rear spoiler. All those changes gige the Rapide S an more aggressive appearance.