I don’t think itachi faster then madara you saying he kept up with naruto and bee but they were talking to him trying to get information about edo, and even if he is faster then them that was KcM- madara kept up with BsM something that’s why faster. The reason itachi used izanami was because they wasn’t going to be able to beat him in sage mode he dodge sasuke arrow with ease at full power so basically that was plot protection so they both would die in that cave plus the only why to catch him in izanami was to get stabbed through the chest he would be dead if he was edo. If we go by with manga say 2 Uchiha with the same eyes(meaning MS) can’t cast genjutsu on each other and that came straight from itachi mouth