They won't be able to get them out because its instant... the minisecond that he's in the genjutsu, could be any time itachi wishes....the bijuu wont have time to even realize they were in a genjutsu..
As asuma told kakashi, One second he was talking, the other he collapsed..
And to take out a jounin on kakashi's level in just 1 use...and deliver almost permanent damage that only tsunade can heal... pretty op..
You have to remember Itachi never wanted to kill him...which is why kakashi started asking why he didn't make it a week, or a month instead of 3 days...
Itachi estimates how much a person can take...for sasuke he normally only does 1 day of torture