Just to get an idea of what the new Full Frame 6K sensor images will look like, we could possibly check out Nikon's new D3X Full Frame Sensor. The sensor in the D3X has almost the exact same specs to RED's Full Frame sensor, with the exception that RED's will shoot up to 100 FPS. and Nikon's is only 5 FPS. But if I where to guess, RED's Full Frame sensor will look just as good if not better, then the D3X, BUT IN MOTION, UP TO 100 FPS!!!
You can check out some 6K stills from the D3X on the Nikon site for an idea.