I am under the impression that DSI can only divert boundary layer air up to the speed of Mach 2, after which the the thickness of the boundary layer will exceed the height of the bump and go into the inlet.Who told you that? DSI is perfect for supersonic flight. The "S" in DSI stands for Supersonic. The intention of the bump is to force out the boundary layer out of the way at higher speeds.
If you like an intake that is optimized for lower speeds, a smaller but open one would suffice, like those on the JH-7A or the Jaguar.