Is there such thing as an EF6 tornado?
No, the maximum rating is EF5. The primary factor in rating a tornado is damage, and since EF5 damage is total destruction there is no room for a higher category. Also, there is no upper bound for EF5 winds; any tornado with estimated winds over 200 mph is an EF5.
There is no such thing as an EF6 tornado. The scale only goes as high as EF5, which is open ended with no upper bound. Most buildings struck by the full force of an EF5 tornado will be completely destroyed. Smaller buildings such as houses and restaurants will be completely swept away. High rise and mid rise buildings will be structurally deformed and may collapse.
Theoretical winds for an F6 tornado are 319-379 mph. However, because tornado ratings are based on damage, the highest a tornado could ever be rated is F5. So the F6 rating is purely theoretical with no applications in the real world. On the enhanced scale there is no EF6 level theoretical or otherwise.
Sort of. On the original Fujita scale theoretical categories F6 to F12 were plotted, but they are never used, as ratings are based on damaged, and an F5 tornado leaves nothing left to destroy. Since it was never used, the Enhanced Fujita (EF) scale did away with it completely, giving any tornado with estimated winds over 200 mph a rating of EF5.