Currently, It is not possible to port a XNA game to SharpDX, because It has several missing features: For the graphics API, no model loading/rendering. No Audio/Input. Concerning the API compatibility, there are lots of areas where there is almost a one to one mapping (game class, content manager, effect, basiceffect, spritebatch, spritefont) and other areas where the naming/organization is different (resource creation, graphicsdevice....etc.). The goal for this 1st beta was at least to have the minimal infrastructure to fully use Direct3D11, as I'm using it to do some prototyping/demo with it (using compute shaders...etc.). Next releases should address progressively these missing features.
How similar is the SharpDX.Toolkit to XNA? Would it require much work to port a game over to SharpDX if anyone wanted to?
But I guess that for a better porting experience, and if you are looking for a one-to-one XNA compatibility, MonoGame is definitely a better option. SharpDX Toolkit is more targeted to users that want to have access to the full Direct3D11 stack.