However I suggest you look at using this stellar lens without TC, and try cropping instead and look at the difference in the out of focus areas, Bokeh suffers here again, these double line type halos really distracts.
where i am (wildlife is not that close - or you can't get that close) cropping is not a very good option especially with a lower megapixel cam like the D4 here.. as the loss of detail is significant and a greater loss than the artifacts introduced by the TC.. so sometimes it is a matter of getting a shot or not and suffering the background artifacts.
i have plenty examples using the TC that were completely acceptable.. so it is hard to know when and where not to use it.. example below with D5 and the new 600/E FL (could be the old lens was more susceptible to this.. as you indicated in the wood duck shots, which was the old 600/G and is used above)
i agree that the cedar above has particularly bad artifacts - which i guess i should clone out when i get around to it. (will post result)
Nikon D5 ,Nikon AF-S NIKKOR 600mm f/4E FL ED VR
1/2000s f/8.0 at 850.0mm iso1800
this shot with new 600/E FL without a TC doesn't have the best background. i will search for other examples.
Nikon D5 ,Nikon AF-S NIKKOR 600mm f/4E FL ED VR
1/2000s f/9.0 at 600.0mm iso1600
bare 600/E FL with similar artifacts.. i should probably start a separate thread on this.
Nikon D5 ,Nikon AF-S NIKKOR 600mm f/4E FL ED VR
1/3200s f/9.0 at 600.0mm iso1000
Nikon D5 ,Nikon AF-S NIKKOR 600mm f/4E FL ED VR
1/320s f/9.0 at 600.0mm iso8000
this one is with the old lens/cam combo and doesn't have the artifacts.. or as much anyway..
Nikon D4 ,Nikkor AF-S 600mm f/4G ED VR
1/4000s f/7.1 at 850.0mm iso2200
here is bare 600/E FL and cropped.. i am not impressed with this shot at all.. for detail or background.
Nikon D5 ,Nikon AF-S NIKKOR 600mm f/4E FL ED VR
1/3200s f/9.0 at 600.0mm iso5000