Focal reports accuracy and reproducibility of focus of different lenses and bodies. On the 7D, the 100-400mm is very poor, which I can confirm.http://www.reikan.co.uk/focalweb/index.php/online-tools/lenscamera-information/
On the 5DIII, the focu is excellent, which again I can confirm. The 100-400mm is in general much better on the new FF.
Adding a 1.4xTC III is no better than increasing the pixels by 1.33x in PS so it is not worth using it with an extender.
My 70-200mm L IS with a 2xTC III on the 5DIII has about the same IQ etc at 400mm as does the 100-400mm.
Trouble is, once you have used the 300mm f/2.8 II with 1.4 and 2xTCs (hand held), you are spoiled for lesser IQ.