The MTF's on the new 100-400 @ 400 are substantially better than the 400 5.6 prime which is a pretty sharp lens. In fact, the charts say it's substantially sharper even with the 1.4x and 2x tele's attached. Didn't expect to see that. If it focuses quickly and tracks well I may have to retire the prime.
I put one of these on pre-order the morning it was announced. Based on the published MTF charts, I fully expect it to be sharper. Like you, I'm looking forward to evaluating the AF speed and accuracy and if it turns out to be fast, my 400 f/5.6 will become redundant.