the physical sun&sky came with maya 8.5 I think. you get pretty fast results in no time but this background shader does have it´s own quality. I mean it looks pretty convincing, infact it is a real good plugin. but you can simulate the same thing with hdri+FG in mental ray. try using Image based lighting + directional light for the sun. if you choose a hdri with slightly bluetones and make the sun color something between yellow and orange you get similar results like with the physical sun and sky. btw if you lighten your scene with an object´s incandecence beware of the expensive rendertimes. better use hdri or a lightDome in combination with FG or GI or both. check out the secondary diffuse bounces on the FG tab also.

good luck!