Is there a such thing as an F6 twister? What would my chances be of surviving one if there is such a thing?
Well, if it happens in Kansas, and you’re in Vermont, your chances are pretty good, I think.
When I first read your post, I thought, “This guys nuts! An F-6 is inconceivable!”
Turns out we were both right. 
F-6 Inconceivable tornado Wind speeds of 319-379 mph
“These winds are very unlikely. The small area of damage they might produce would probably not be recognizable along with the mess produced by F4 and F5 wind that would surround the F6 winds. Missiles, such as cars and refrigerators would do serious secondary damage that could not be directly identified as F6 damage. If this level is ever achieved, evidence for it might only be found in some manner of ground swirl pattern, for it may never be identifiable through engineering studies”
In a word, no. They are measured on the Fujita (I think) scale, and it goes from 0 to 5. You can survive any tornado if you act sensibly, but if you want to capture on tape what an F5 does to the swingset in your backyard, then no.
Just a little hijack here:
Is it “tornados” or “tornadoes”? The plural of tomato is tomatoes, isn’t it?