2x Converter and 300

It appears that I can use a 2x teleconverter on my 300mm F4L but in doing so I will loose autofocus....

Has anyone tried this with birds? Is it enough to get good shots, texture etc? ...I would sure love the extra reach but really like the versatility of the 300mm as I can make it 300, 420 ...or possibly 600, plus a 1.6 crop added to all of those, not to mention a good macro.

But are the shots really worth it for birding? I have seen images of 300 F4L using a 2x but the images are all of cards or other objects that do not have feathers or fur.... all information welcome,jim

It will accept the teleconverter but your maximum aperture will drop to f/8 which will probably stop your camera from autofocusing (unless you're shooting a pro body) as well as giving you a noticeably dark viewfinder.