Even Kakashi admits he wouldn't be able to take on Itachi 1on1. Proved by that page from the chapter I posted above.
However, Kakashi's stamina would not allow him to keep going with Itachi. Even when using the sharingan to incredible levels, Itachi can keep going for quite a while. One mangekyo(which, at some point in the battle, I imagine he'd have to use) and Kakashi is slowed down a good bit.
And of course Kakashi was considered a threat to Akatsuki. He was Konoha's 2nd greatest ninja behind Naruto in terms of ability, and Kakashi could probably take on all Akatsuki members 1on1, and win, short of Madara/Pain/Itachi. Not to mention, a huge reason why Pain said what he did was due to Kakashi's intelligence. While yes, his fighting ability would prove very troublesome, his intelligence is what could have came back and bit Akatsuki in the ass.