These test images are to show real world usability of files shot at high ISO settings on the Canon 6d. No noise reduction was performed on any of the above images. All images were shot handheld with the Sigma 50mm 1.4 Art lens. Increased sharpness over the image shot at 12,800 can be attributed to the aperture being stopped down from f/2 to f/3.2. The sharpness and contrast of the Sigma 50mm 1.4 Art at f/3.2 (f/2.8-f/11) is really incredible, and it's no slouch at wider apertures either!
Instead of using high ISO camera settings I often use a wider aperture, slower shutter speeds, rely on image stabilization (when available), or use a flash. I think these results are quite good, and will not hesitate as often to take advantage of (much) higher ISO settings in the future.