I am interested in the argument as to why Kamui would kill Itachi so easily. As far as I know, Kamui has only been used on linear targets, and when it was attempted on a dynamic moving target, Deidara, it missed. If Kamui were that powerful, why didn't Kakashi use it to instantly kill Madara in the war? I understand why Kamui was not used against Kakuzu (although Kakashi could have arguably used Kamui on Kakuzu's entire body), but if Kamui is so strong, why was it not used against Hidan?
Therefore, if Itachi were to lose, it would not be because of Kamui unless someone can demonstrate its speed being enough to capture one of the notable speedsters of the manga, and also explain why Kakashi rarely attempted it against his other opposition in the manga. At the very least, it would be Kakashi laying a trap for Itachi which would allow him to land Kamui.
Regardless of the intricacies of Kamui, with full knowledge, Itachi should be able to kill Kakashi without much effort without Susano'o. Susano'o is Itachi's absolute last resort, and he didn't become a highly regarded Kage-level shinobi through its use, so he is able to stand on his own merits as an extremely powerful ninja without it, even capable of bringing part 1 Kakashi down with a single technique and walking away, and even escaping a Sannin without being killed.
In terms of speed and skill, Itachi is tiers above Kakashi because he was a genius in every aspect. Itachi might be a tad lacking in the intelligence and stamina department given it is War Arc Kakashi, but he will be able to make up for it with full knowledge. Itachi will know that Kakashi can outlast him and can fight accordingly. Kakashi will conclude that a battle of attrition is best, but Itachi's high level techniques and superior genjutsu and speed would realistically overwhelm Kakashi making outlasting Itachi unreasonable.