The Clearcoat layer in the new Principled BSDF should - according to the Disney Paper - behave like an additional Specular layer with ior of 1.5 when set to 1. But it behaves very strangely. When Roughness is set to 0, the Clearcoat looks much darker than the Specular layer in comparison. And when you increase the Roughness of Clearcoat above 0.02(!) it gets even much darker. Also, when using metallic and setting the base Roughness to something other than 0, the Clearcoat Reflection gets colored - which it definitely shouldn’t (“The secondary lobe represents a clearcoat layer overtop the base material, and is thus always isotropic and non-metallic” ~ Disney). Only when increasing the base Roughness much more, the Clearcoat reflection seems to become white again. Here a video demonstrating the bug /strange behavior.
At the moment - at least for me - the Clearcoat seems to be useless because in most cases it’s not even visible. You can increase the value above 0, but it looks like the layer gets “added” like with an Add Shader(?) so you can’t do that without breaking energy conservation.
Here a video demonstrating the bug:
And here an image showing how much darker the Clearcoat layer is then the Specular layer: