I must be in some utter minority in thinking he didn't speak of actual bad eyesight. In fact, the whole idea didn't occur to me before I read this thread. When watching the episode, I thought Kakashi was just reminding Itachi that he's not the only one with a sharingan and that it's Kakashi specialty to copy techniques. Referring, metaphorically, to eyesight would suit an Uchiha since their ultimate jutsus depend on special eyes.

But now after reading this thread maybe Itachi indeed is risking his fancy eyes every time he visits a week's worth of torture upon someone in a second.