I'll never forget the day that I moved from the 400/5.6 to the 500/4-IS. I felt guilty and like I was cheating. The IQ of those first images blew me away. Now, my technique is even better and I'm constantly astounded at the IQ, even with the 2.0X TC-III attached.
I grabbed the image below, showing a contestant in the bike portion of a triathlon that went by as I was shooting birds. The sharpness, contrast, color and quality of bokeh are world class, IMO. I laugh when I see some dumbass saying, "I don't use a TC on my lens because it degrades the IQ too much." How much frigging IQ do you want??? For me, nothing beats pixels on the subject, particularly when you're talking quality pixels, like in my 5D MkIII:
Biker 2 by
dcstep, on Flickr
That's 1,000mm hand held at f/8, 1/1000-sec., literally a grab shot of somebody passing by.