The lens is fairly new, I love it. These are difficult shots due to the distance.
Reported Settings. These are what is recorded in EXIF and, in the case of stacked TC's the f stop is wrong. I used AV setting of f/11, but the aperture of the lens would have varied according to the TC with it being stopped down more without a TC
Also, it was a winter day and the light varied from shot to shot so the shutter speeds varied according to available light.
No TC - 1/2500 sec f/11, ISO 800
1.4X - 1/3200 sec f/11 ISO 800
2X - 1/3200 sec f/11 ISO 800
1.4 + 2X - 1/2000 sec f/11 ISO 800 (not the actual equivalent aperture)
It might be interesting to redo them with my 1D MK IV, but in our near 100 degree weather, the air is too turbulent, and the 1D MK IV is going to be sold soon.
Another thing is: Did you scale the images made with the TC or change the focal length? Comparing the length of the tower by hand doesn't give a magnification of 2x or 2.8x compared to the 1.4x (maybe this is also done on the webpage). I ask this because scaling images is also often problematic and there is quite some discussion on how you should scale the images in order to provide a "fair" comparison.
What we can conclude however is that taking images with a TC is much more complicated than using the bare lens.
They are crops, so the scaling cannot be measured when you crop a image. As noted, I cropped them to show the tower at about the same height, except that the no TC crop resulted in a 100% crop so it is a little smaller. If the objective is to see which combination produces the most detail of a distant object, a crop works best.
Full images are linked below to avoid taking up bandwidth for users.
1.4X + 2X
A 50mm shot for comparison. Try scaling it