Will they autofocus with the 100-400 Is zoom?
Only if they are mounted on a camera body which will autofocus at the effective aperture is the simple or not so simple answer.
Take the 400 end of the 100-400 and stick a 2x Extender behind it it becomes an 800 mm f/9.5 lens.
I think that none of the present range of Canon DSLRs will autofocus at this aperture. Perhaps they will in future.
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