A few weeks ago I was able to test a Canon EF 200-400mm F/4L IS USM extender 1.4x thanks to Robyns. For some time now I am looking for a zoom lens, type 100-400, to use along the 600 prime. I don't find he present Canon EF 100-400mm f/4.5-5.6L IS USM good enough, the IQ is far behind the IQ of the 600, which is of course not surprising seen the difference in price. That's why I am interested in the 200-400.
Sharpness: superb at 400 mm (comparable with my 600 mm), second to none at 560 mm (with the build-in converter).
Bokeh: more dan adequate. At 400 mm however you clearly see the effect of the size of the diafragm. The bokeh of my 600 is smoother but the bokeh of the 200-400 is not too bad, far from.
Minimum focus distance: 2 m !!!My 600 needs 4.5 m. Imagine wat 560 mm can do with butterflies and dragonflies at 2 m distance !
AF: speed is OK, but AF is hunting with small subjects barely covering the focus point. Once the subject is slightly bigger, the AF is fast and accurate.
Advantage: zoom capability and the build-in converter which can be engaged and disengaged at the blink of an eye.
Disadvantage: hardly any unless that the lens is still a big one (although not as big as the 600) and heavy so not really an option as a second lens next to the 600. I cannot see anyone lugging around with these 2 lenses .
Superb telelens with a lot of qualities and hardly any disadvantages. If you don't need 600 mm F4 (and also no 840 mm F5.6), this might be your lens. But it's too heavy/big to use along the 600 prime, at least for my taste. So I will wait and see whether or not Canon announces the long awaited new 100-400 at Photokina 2014.
A few test pictures: