The review I mentioned earlier was indeed in the Professional Photo Magazine, and compared the Canon 100-400 Mk2, Nikon 80-400VR, Nikon 200-500, Sigma 150-600 Contemporary, and the Sigma 150-600 Sport.
The lenses were graded on their Features, Handling, Optical Quality and Overall Score.
The Canon scored 5/5 in all categories
Nikon 80-400 scored 5/5, 5/5, 4.5/5 and 4.8/5 respectively
200-500 scored 5/5, 3.5/5, 3.5/5 and 4/5 overall (The least sharp of all the lenses)
Sigma Contemporary scored 5/5, 4/5, 4/5, 4.3/5 overall
Sigma Sport scored 5/5, 3/5, 4.5/5, 4.2/5 overall
The clear winner for Nikon users in terms of IQ is the 80-400, sharpest in the centre wide open