Incorrect, the features DX11 allow over DX10.1 (I take it that is what the other supports as ATI loved that version) can ammount to more than just the 5-10% the hardware shows.
The major change is in the generation of model geometry. DX11 games do not have to send vast ammounts of vertex data to the graphic card and also does not need to store vast ammounts of vertex data in the graphic card. The result is less memory used, less bandwidth used and even less CPU power used (although the GPU probably is slightly more loaded). Also DX11 allows games to more efficencly utalize GPU power by simplifying distant geometry without having to resort to loading new models while improving the detail of art closer to the camera.
However keep in mind that this is only when running fully DX11 designed games. In DX10.1/10 the differences start to become like BlargHonk described.