Specialized uses a different blend of carbon for its S-Works frames (11r) versus non S-Works Frames (From 6r to 10r). There are weight and stiffness advantages to the higher level carbons.
And the differences can be measured. An 11r frame will weigh about 150g less than a 10r frame. (So it is not a question of how long is a string.)
As for stiffness, I don't know that actual measurements exist, but having ridden several versions, I personally would not buy anything less than 10r.