Hair with any detail indeed can have huge negative effect on FF performance, and that's one of the points where SF stands out. 

For a render of a plain plastic doll the difference would probably be much smaller. It is not for nothing that plastic doll scenes are the traditional game. The question how much difference there is is best answer by him who poses it. Each tool has its application. If SF is not good at this let us know and it is good we still have FF on backup.

For my work, unless I need microfacet displacement, I do not even THINK of ever going back to FF.