After a couple of weeks of experimentation my findings on the Yongnuo 2x III are:
Positives
1. Very good performance for the money, almost as good as the Canon II and yet cheaper than a used example.
2. No discernable AI Servo issues, focus seemed as quick and accurate as with the Canon and actually suffered fewer 'hang-ups' when the Canon refused to try to focus until nudged with MF. My main targets were aircraft and occasional passing birds.
3. Well-built.
Negatives
1. Can't quite match the Canon II at centre-point so likely to be worse than the III, though at less than half the price.
2. Occasionally my 1D4 became confused as to minimum aperture, showing 4.0 when the Yongnuo was attached ( actual minimum being 8.0 ) but exposing at 8.0 when shot ( per the demands of physics! ).
3. Apparently lower light transmission than the Canon II which results in darker exposure.
4. Not weather-sealed as far as I could determine.
5. Less internal flocking than the Canon II.
Still a great buy. I'll probably hold on to my copy as a back-up, as I use a 2x every day.