I bought a used 600mm Sigma lens this morning for $135, and thought I'd compare it with my 100-400mm L + 1.4X TC. First, I took several shots and found that in the dark windy rainy weather that a 1/1000 sec image took a very high ISO setting on my 5D MK III, such that the image was noisy. So, I mounted the lens on a monopod, held my breath, and set the shutter speed to 1/320. My tripod was in the studio, and I did not want to brave the weather to go out and get it.
Then, I used the same setup with my 100-400 + TC. I could immediately see the difference thru the viewfinder, but went ahead and took the image for a first shot at comparing them.
To be fair, $135 versus almost $2000 worth of lens is a bit extreme, but I wanted to see the difference. I may have to get out that tripod after all, because there was a lot of difference, and some of it was movement.
Here is my first try, with images cropped and sharpened in Lightroom.
Canon 100-400L + 1.4X MK II TC
To be fair, if you were printing the images at 8 X 10, I doubt that most people would see the difference, they look pretty good at a small size.
I'll try again later with my heavy duty tripod just to compare.