I shot a careful resolution chart test last night, using the “Live View” feature of the Canon EOS 1D Mark III to ensure perfect focus for both lenses, which excludes autofocus as a source of error.
While I haven’t completed my analysis, there is no question that the “II” version offers a stunning improvement to the clarity of the corners at the wide end (16, 20, 24mm) and a modest improvement at the long end (35mm). See for yourself:
16-35mm f/2.8L (left) and 16-35mm f/2.8L II (right)
extreme corner, 16mm @ f/2.8 (wide open)
It’s not just one corner that looks like this, it’s all four corners. The “Live View” feature guaranteed spot-on focus, so it’s not focus error.
Performance in the center is close, but at least at the wide end the new “II” lens seems to offer slightly higher contrast. This is on the 1.3X-sensor in the Canon EOS 1D Mark III; I haven’t yet tested on full frame (EOS 5D). Of course, it could be curvature of field causing the reduced corner performance with the 16-35mm f/2.8L (v1); I haven’t definitely ruled that out yet. But that seems unlikely, given the consistent performance at different focal lengths.