The king on a 6 might have a problem when you open the door.
But even then if you open it slow I doubt if it will be much of a issue.
I'm not sure I get on board with that. There are enough of us running the princess on a 6" and we complain about smoke spillage when it's warm. I have a feeling a king on a 6" flue on the warm 40-50* days may be a smokey mess.
Mine has 27' worth of liner with about 3' of double wall off the stove so around 30' total. For my non cat it was too much chimney for my BK I get smoke spillage if I'm not careful on warmer days(probably 40*+)