I updated the falling cubes demo to separate out the rendering and physics into separate WinRT components - see the WinRT subfolder under http://github.com/taumuon/Taumuon.Game . The performance of this wasn't noticeably different when consumed by a WP8 DirectX application, but I then switched over to a "DirectX with XAML application", so that the application was hosted via C#, and consumed the WinRT components via a DrawingSurfaceBackgroundGrid, and performance fell off a cliff.
I created the C# host as I intended to put as much game logic as I could into an F# portable library, and the only way to do this is to have a C# hosting application assembly as WP8 apps, unlike Windows Store apps, don't allow WinRT(P) components to be created in anything other than C++.
Instead, of fighting the performance issues, I decided that it would be better to implement the game logic purely in C++ - see the WinRT subfolder under http://github.com/taumuon/Taumuon.Game . This replaces most uses of the C++/CX extensions with standard C++ types, and implements picking, loosely based off http://halogenica.net/ray-casting-and-picking-using-bullet-physics
It gives a childish sense of satisfaction to do something as simple as destroy a wall of bricks so that the wall collapses.
Once I get time, the next step is to make the wall collapse more realistically by adding spring constraints between adjacent blocks, and to remove those springs once they are stretched beyond some elastic limit.