The beauty of the 400mm f 5.6 lens is that it is so light you can hand hold it with no problem. I'm not against the sidekick, I have one myself and use it in heavy lenses. Why do you need one finger control of a light lens?
Not being one of you male types with the upper body strength, I find it all I can do to hand hold my 7D/100-400 combo at 1/400th. When the speeds drop below that, and they often do because that lens is a light hog, that one finger control is verrrrrrrry nice.