The F-Scale actually goes all the way up to like F12 or thereabouts.
But the highest tornado rating ever given is F5. The reason, an F5's damage is so complete that you couldn't distinguish higher wind damage, even if it existed. An F5 will pick a well built home off it's foundation, disentegrate it, then scatter it around the neighborhood, leaving a bare foundation. As you can see, there's not much more damage that could be done at that point, so there would be no way to estimate higher wind speeds based on damage.
Also, that 318 mph top wind speed that was measured by The Doppler on Wheels team (and often cited by the media) doesn't necessarily mean the tornado was an F6. That windspeed is right at the top of the F5 scale. But this windspeed measurement was taken several hundred feet above ground, not at ground level. In a tornado, the winds are always higher above ground level than at ground level. That's why they recommend lying in a ditch or low spot if you're caught out in the open. A tornadoes wind speeds are less the closer to ground level you get. Friction between the tornado and the ground are the reason for this.