its more than just having sharingan though.....
i think it has to be at a high enough level to be able to perform some of the more advanced stuff.....i havent been following the itachi conversations much to know all those details about mange and um...that T thing...lol....but i would think of it this way.....if kakashi was to steal say, sasuke's eye the way it is now instead of the one he already has, then i would think his abilities are limited to what he can do with the 2 pupil level that sasuke is at. Since he's not uchiha, his sharingan ability immediately plateaus there, and i doubt he'd be able to develop the 3rd pupil with it. If this is the case, then there wouldnt be a way for him to develop his current eye to reach the level of itachis (and even though both him and itachi have 3 pupils, i would be surprised if itachi's wasnt more developed in a way that just isnt as easily distinguishable)