I don't think it makes much sense to talk about just learning the notes. I assume you don't mean just theoretically knowing the notes in the piece, but actually playing them. You can pick a stretch piece to improve your technique, but La Campanella isn't likely to do that -- it's more likely to make your technique worse in a few ways at least.
I would suggest looking at pieces at around the RCM 10 level at most if you're looking for pieces to stretch your technique.
That said, the question really becomes what you mean by "learning" the piece. You are unlikely to be able to play it at performance level, or even play the notes at tempo, anytime soon. If you really want to use sections of it as mini exercises of sorts, I suppose you could do that, although it may be counterproductive because you don't know what the actual technique should feel like.
You don't just learn how to play by learning difficult pieces. You could still attempt parts of the piece, I just wouldn't expect to get anywhere near finishing it.