Where mine really impress me is the flat sharp field. The two I have are amazing. A good flat sharp field with little or no distortion. Which is very important to me. Close quarter shooting makes the lenses a perfect match for me. Bare lens my 300 is so sharp it is amazing. Shooting with it everyday I can immediately see the quality drop when I put on my 1.4. Not only that with the 1.4 on a bigger lens it slows me down to a crawl. Af speed, Long lens technique has to kick in to get sharp shots. Just not fun for me. The 1.4 on the 300 I find acceptable. It is light and IS does a good job. The 2X is strictly a no go zone for me with either lens.
Kinda curious about the new 200-400. Looking at the MTF charts. I personally do not think I would buy one. For me that is. Again, not for most I guess.
That is just me. It looks like it is not for most. So no sweat.. But that is the way mine work, and the way I like to use them for my style of shooting.