I thought I would have a go at comparing the new ‘GGX’ Specular attribute in Arnold against ‘Beckmann’ (default).

“GGX is a microfacet distribution. It has a sharper peak and a larger tail than Beckmann. GGX is suitable for modeling light reflection from surfaces more realistically.”

It seems to give a subtle softer highlight that falls off more gradually. The effect seems more obvious on the brighter specular highlights on the metallic surfaces below.