You linked to scaled versions of the images but i found the originals in your gallery
It's pretty hard come to a definite conclusion regarding the quality. Also at the shot with the 2x converter a cloud is on the main tower reducing contrast and it seems like some kind of noise reduction is employed. Additionally there may be air turbulences and slight focusing errors etc.
I think most people will be happy with the 1.4x extender on any tele lens F/4 or faster but the 2x extender is only a compromise.
A 50mm shot for comparison. Try scaling it
That's quite impressive!
Yes, there are flaws in the test, a distant object miles away is not really the best type of test, that pesky air, clouds, water vapor, etc change all the time. Also, from a wooden deck, someone walking in the house might affect it.
Its pretty clear that a 1.4X does a good job with a 10mp 1D MK III. The result with a 18mp 7D might be different, I might setup on concrete one day and try that. I now have a laptop holder on my tripod, or can use a long USB cable and focus / capture remotely to get the best possible image. However, something closer might be better.
I really do not use my 100-400mm L with TC's, its too much of a bother for any possible gain. I can crop most of the time, and other issues usually out prioritize using a TC.
Here is a cropped image of a female redwing blackbird as she dropped from our crabapple tree to the ground to pickup sunflower seeds. I loved the way I could AF on the bird right thru the clutter of the tree with my 5D MK II. I always was wondering at those who could not AF with it. 100-400mm with no TC.