In a paper published recently "http://jcgt.org/published/0008/01/03/" they cite an imageworks paper in which they discuss improvements they made to the principled (disney) shader to include better energy conservation, and analytical multiple scattering micro- facets. It's a pretty good read and covers at least the energy conservation byob brought up earlier regarding blenders implementation of clear coat.
From the original Disney paper, that Dade linked earlier:
For our clearcoat layer, we use a fixed ior of 1.5, representative of polyurethane, and instead allow artists to scale the overall strength of the layer using the clearcoat parameter. The normalized parameter range corresponds to an overall scale of [0, 0.25]. This layer, even though it has a large visual impact, represents a relatively small amount of energy so we don’t subtract any energy from the base layer. When set to zero, the clearcoat layer is effectively disabled and incurs no cost.
