I started out with the 70-200 and the teleconverter and wasn't really impressed. Eventually I got the 300 f/4 and it is a much better option. The 300 with the teleconverter is superb as well and works as a great closeup lens for insects, butterflies, and flowers. But as others have mentioned you can't use the same Nikon teleconverter on the D and AFS lenses. I have no experience with the Kenko.
The thing is, while you are not shooting lens charts, real world objects need to be sharp as well. And you can see the difference in normal sized enlargements. Stopped down 2 stops the combo was much better, but who wants to shoot at f/8 to f/11 (effective) for birds and animals.