Sounds good for the layering system can't wait! But so far as I know ABC shader is more recent then GGx no? here it says : "ABC" is a brdf model proposed in 2012 by Joakim Löw, Joel Kronander, Anders Ynnerman, Jonas Unger
Here a link to a guy that made a plugin for vray, look at the tail, very cool! you can really adjust it for you own need :)
http://www.shlyaev.com/rnd/37-cpp-category/55-abcvray so you can create like many reflection lobe with only one layer of reflection, it's more accurate that multilayer and it should render faster then multilayers reflection.
As he says : ABC has sort of separate controls over specular peak and tail. It is possible to have sharp peak and long tail with this model like those in chrome metal.
Let me know what you think, GGX is good, but this one feel more.. next gen if you want, ABC can do what GGX do bbut GGX can't do what ABC do and I really think Corona should be at the forehead of everything and get the best. No one would complain of one more shader model in Corona :)
And since the mentality of Corona is to have a simple and straight forward workflow, I think it suit it perfectly to get that nextgen shader (before everyone else.. oups I think renderman already included though ^^ ) because with that shader we don't really need ( maybe rarely in some special case) us to have multi layers to get what we want. To me, ABC BRDF is the best so far.
,t