I currently use the 600II, 400II, and 300II. They are already stunningly sharp even with the 2x.
What troubles me is that the released MTF data on the III's appears to offer less resolution, not more, over the II range. Pay more get less?
The other question is of course performance on the upcoming mirrorless future.
Makes me wonder about a Sony A9 and their new 400f2.8