I don't disagree with Dave's recommendation that you'd be better off with a longer lens rather than putting a teleconverter on your 80-400. But, as a hobbyist myself and 80-400 owner, I sometimes use a Kenko 1.4x teleconverter with my 80-400 and it works adequately. The image quality is good, though I'm sure you'd do better with one of the Sigma zooms Dave recommends, and it autofocuses OK in good light on my D300.
You say you can't justify the expense of a longer telephoto (that's what I was thinking too), so you might try the Kenko. It does work, and you can get usable images with it. But, in anything other than bright sunlight you will notice it can have autofocus issues.
The 80-400 with the Kenko 1.4x is fun to use and you'll probably like the images you get with it (there has been a debate on this, though, as to whether you'd do just as well using the lens without the 1.4x and cropping). But particularly because of autofocus issues in anything but good light, you shouldn't think of this as a good substitute for a longer lens.
One reason I'm not inclined to trade my 80-400 for a Sigma zoom is that I like the 80-400. It is relatively small and light (for 400mm) and the zoom range is useful in many situations (without the teleconverter).