This idea that SR and MMR are the same thing, or that one is an ‘extension’ of the other, is total nonsense that Blizzard/Activision reps have said to keep us from understanding either system, and our terms of use for competitive play.
The simple fact of the matter is that competitive play is handicapped by the Matchmaking engine. Competitive players should not stand for this, and they should demand transparency. More info in my thread on the subjects of SR/MMR/handicapping: