Hi Joshua
I recently acquired the canon 100-400 Version 2 and after 2 or 3 casual outings, I would say that it is every bit as sharp as the 400mm F5.6L prime and better (pixel peeping) than my 300mm F4L prime. The AF on the Version 2 100-400 is noticeably faster than my 300mm F4L and easily matches the 400mm prime.
In my opinion, the version 2 100-400mm can replace 3 lenses; the 70-200mm F4L, 300mm F4L and 400mm F5.6L. However, the V2 100-400 is noticeably heavier than the 400mm prime…
I have not yet used the V2 100-400mm for astrophotography so I cannot comment on how it would compare to the 400mm F5.6L prime (21 lens elements compared to 7).
Here are some very recent sample shots with the 100-400 showing exceptional IQ and resolution.
Cheers
Dennis
EDIT:
I mistakenly posted Eastern Water Dragon photos taken with the Canon 100mm F2.8 macro lens. I have replaced them with the ones taken with the 100-400mm Version 2.
The 4 stop IS on the V2 100-400 and the closer close focusing distance compared to the 400mm were a strong input into my decision to buy the new 100-400.