DirectX has massive overhead compared to low level coding. This isn't debatable, you're either aware of it or not. Simply looking at how consoles perform compared to a PC with similar number crunching power running DX or OpenGL it makes it overwhelmingly obvious that it's a pretty plausible best case prediction. A computer running Windows with double the CPU and GPU power of an Xbox 360 will run Skyrim or Bioshock Infinite at the same settings at a lower FPS than it does entirely because of DirectX inefficiencies.
We don't really know just how much control Mantle will give game programmers but if it is as much as they get on consoles then double performance is definitely possible if they put the time in.