What lens is it that you have now?
I can't really recommend a TC on any xxx-300 zoom lens, unless you have a Sigma 100-300 or Sigma 120-300. The variable aperture consumer zooms arn't worth it.
Sigma 50-500 or Tamron 200-500 - or if feeling flush Sony 70-400 SSM?
If you do want a TC, then the Kenko Pro 300 1.4 DG will autofocus with all my lenses, even my Tamron 24-135 (which is f/8 on the long end)
Sony 70-400 SSM thats the very nice very expensive lens you just bought off StanTheMan I believe. (jelous)