iRay itself does seem to support moblur in MAX at least. Perhaps it's just not implemented inside DS as yet (or they disabled it for some reason ?) Here's the MAX manual on it:

http://help.autodesk.com/view/3DSMAX/2015/ENU/?guid=GUID-25648CF2-A60F-450F-8880-8ADCD2EB13FF

Motion blur is not a physical based render feature, this is a render trick in the renderers we are used to. The physical based render model is a shift in how things are set up, lit and rendered, so some things will no longer apply. In the case of PBR, this is done in post.