This is my first finding on the hair shader. It indeed improves messing with specular values that Im showing in this short video:
UE4 - Character Customizer - hair shader discussion (not a tutorial) - YouTube video do show some nipples, so if this offend you, do not watch!!
I need to do some hard debugging in one of my project and I will get back with this soon.
I took the freedom to show-off the video (all as WIP for the @mlindborg ) at twitter and also point to this thread. If you think I crossed some border, just let me know.