In the original Disney implementation, the clearcoat layer is just added ontop of the material without properly “mixing” it in by also substracting energy from the underlying shader.
“This layer, even though it has a largevisual impact, represents a relatively small amount of energy so we don’t subtract any energy from thebase layer.” (Physically-Based Shading at Disney 5.4 P16)
In their implementation, this is done for performance reasons. Is there a performance benefit for doing it in Cycles too though? The problem is, that the Principled BSDF allows pushing up the Clearcoat value to > 1.0, which makes it obvious that the Clearcoat specular is added instead of mixed and breaks energy conservation.
I know you shouldn’t do that anyway, but in this case the value should be either clamped at 1.0 or mixed instead of being added to make the result less incorrect and keep energy conservation.