All the quantitative measurements show that at 200mm there is nothing to choose between the 70-200mm f/4 IS and the f/2.8 IS II. At 70mm, the f/2.8 is nigh perfect sharpness at the centre whereas the f/4 is very good/excellent. On Sunday, I AFMAd my f/4 with the 2xTC III. At 400mm with a real life target (a very exciting brick wall), it wasn't much different from my 100-400 L on the 5DIII. But, everything is relative when discussing sharpness. The f/2.8 300mm II is so much sharper that it blows the the two zooms away - you can rezz it from 300 to 400mm in PS and it is still far, far better at picking out fine details and contrast.