Ian,
Check lensrentals.com: they have MTF charts for all (most) tele lenses. I look at the sigma zoom MTF charts from another thread here, and like typical zoom (not offending an manufacturer here), it is soft compared to the fixed focal length lenses. So if soft and slower AF, then one has to weigh those factors against continuing to save for a better lens (again independent of manufacturer).
In general, telephoto zooms have been softer and AF slower, but we can hope that will change with new designs. The Nikon 200-400 narrowed the gap between zoom and fixed FL lenses; will the new Canon? We need the gap narrowed and the price reduced.
Roger