The Canon 100-400 is not the sharpest. The new Canon zoom lenses like the Canon 70-200 f/2.8L IS II are in a totally different league. Unfortunately Canon has not made a replacement for the 100-400 except the very expensive 200-400. On Nikon I'm not too impressed with thte 70-200 f/2.8 VRII and cannot imagine that a 2x extender will make you happy. Remember also there is a loss of two stops, of course. If you look at critical sharpness I always find that extenders are a compromise that can be accepted but with a cost in details. I have a Canon 500 f/4L IS which although it is not a mkII is still pretty sharp at f/4. With an 1.4x mkIII extender it's still good, but a 2x extender it is not worth using. The bokeh suffers tremendously. I have not used similar Nikon lenses but would be very surprised if it is not very similar. Zoom lenses are usually worse. So I would not speculate on using extenders and go for the new 80-400 if this is the need.
Thanks for the advice. I bought my Nikkor 80-400 some time ago, mainly for use with my D7100. I also bought a Nikon 1.4x extender, just in case it was able to offer a worthwhile improvement in detail.
I recall doing some rigorous comparisons of static targets to confirm that the extender really was capable of producing slightly better detail when compared with the the same scene without converter, after cropping to the same FoV.
Then I got involved in the complexities and variability of AF Fine Tuning, and how its accuracy might vary with focal length and distance to subject, and so on. I guess I got so exasperated with that process, I've put the extender issue to one side and haven't used the extender for a while.
I suspect there are unavoidable trade-offs when shooting moving subjects, using an extender, which might completely negate any benefits. For example, I understand that the DoF of a 400mm lens at F5.6 should be the same as that of a 560mm lens at F8 (400x1.4=560). That's fine. However, if shutter speed is important, one has to increase ISO when stopping down, or underexpose, thus increasing noise.
Furthermore, in order to get more detail in a moving target when using a longer lens, one should ideally be increasing
shutter speed, not merely maintaining it.
The following cropped shot taken recently on a trip to the seaside was at F8. ISO 200, and 1/1250th, using the Nikkor 80-400 at 400mm. Would any purpose have been served adding the 1.4x extender and using the same settings, apart from the possible effect of a shallower DoF?
In order to ensure the same DoF, I would have needed to use F11, ISO 400 and 1/1250th. In order to retrieve the slightly greater detail that a 1.4x extender can provide, I would perhaps have needed to also increase shutter speed and further increase ISO, thus introducing more noise.