Genjutsu off. But yeah, I think Itachi's stamina is the most important factor. We don't know healthy Itachi's level of endurance. It can be presumed as superior to his sick self but how much more is hard. Mangekyou drains a LOT. Before Sasuke learned EMS, he couldn't use the MS for very long even though he wasn't sick and his eyes would start to bleed.