I would think it would be a fair to compare MTF charts from recent Canon lenses to formulate an idea of how the lens will perform.
Compared to the 70-200mm II the 100-400 II chart looks better at 400mm than the 70-200 II at 200. I think there will no longer be a 70-200mm with 2x or 100-400mm debate.
It looks better than the 70-300mm L, leaving the only reason to have that lens is weight and economy.
The only reason to buy the 400m F/5.6 will be the additional cost of the 100-400mm.

The 100-400mm II looks promising so far. I doubt I would want one though.