You Underate Konan really Hard. I can explain how she defeats the Akatsuki 1 by 1 if you like.
& people are too foolish to understand that Pain has no counter to Itachi's Genjutsu. Period.
Pein, or Nagato? Pein does.
It's highly unlikely that Itachi will capture all six paths in a genjutsu, the summons, and what not. All at the same time. Several ways Genjutsu can be evaded and detected is when one path is put in a genjutsu and sees a different outcome then the others. Nagato can easily pick up on this and have a path break the Poor Genjutsu'd bastard free.
OR, he could alter, slow down, or stop sending chakra. This would itself free the Path from genjutsu. It would be left vulnerable, but no more vulnerable then it would have been in a Genjutsu. Genjutsu isn't something a Path has to worry about. Unless it's the Frog Song, or tsukuyomi (Debatable).
All in all, Genjutsu can be picked up fairly easily and Itachi, even if he tries to put a single path in it. Would be swarmed by others or one of the large summons that the Animal Path can produce, before he can make any useful arrangement out of the stalled Path.
Unless it's in Amegakure and it's a similar battle to Jiraiya vs Pein. Then Genjutsu could be handy. But only a select few can beat Pein in Amegakure. the Naraka Path, when hidden, can just continually repair and resurrect each defeated Path.