Depends; DX11 includes a path that optimizes some functions, which leads to some speed increases. DX11 also includes some new features which are very preformance heavy. So on a game-by-game basis, it depends. For example, Dirt2 looses a lot of performance in DX11, but Stalker actually gains performance.
Yes, they all have a DX10 or DX9 path. DX11 is simply used for extra effects. Right now DX11 games simply have DX11 bolted on rather than being designed with DX11 in mind. Since game development takes a while it will be at least a year before you see games that start to really take advantage of DX11.