Not sure of the wear factor yet, but something to keep an eye on.  The "bruxer" type of crown idea.  With my experience thus far, e.max is definitely a reliable material for the challenged occlusal situation as long as there is adequate ceramic thickness.  From what I am hearing in the industry, part of a material's wear compatibility factor is defined by the smoothness and polishability of the material.  Zirconia is a very smooth material.