I don't see how Williams Syndrome could be a spectrum, since it's caused by the deletion of specific genes; they're either there or they're not. I do recall one writer (perhaps Simon Baron-Cohen?) speculating that there may be a disorder spectrum opposite autism which has received little recognition because society is much more accommodating of people who're socially fluent but analytically inept than the reverse.
Link: abcnews.go.com/Health/friendly-extreme-meet-kids-adults-williams-syndrome/