With modern cameras focusing at f/8, the EF 100-400 plus 1.4TC is actually very similar to the 150-600 cameras. Especially since they are not true 600 mm (or 400 mm for that matter). As long as you have a body that can focus at f/8, this really isn't a hole in the lineup.
At infinity focus, you should get pretty close to the rated focal length. It's true at close focus some of the lenses are not as long as expected... Unless you're referring to something else I'm not aware of?