That's good that its coming, IMO I think this should be number one priority, when you have different pieces of a mesh unless you want to separate them before substance painter then reimport, which when I reimport the model put back together so to render with iray, the painting is in all the wrong places, yes same UVW's.
But the main reason this should be number one, is it will save ALOT of people hours of pointless work.
I was really hoping this update would be it, oh well.... I'm just glad to see its coming...