LeBlanc's whole gimmick was destroying a single target quicker than you can blink. While I'm not quite sure she's up to that speed anymore, I'd rather see her ap ratios re-looked at then see her ult completely refresh if she kills with it.
Considering her ult deals 30-40% bonus damage on the last spell cast, refreshing her ult upon a kill would make her op, and they'd nerf her ratios on the other abilities to make them useless.
I agree she should be looked at and tested because you really don't see her picked often enough. She was made to be a one trick pony, and she still is, but now it's like she's doing half the trick and it doesn't give the same 'ooh and ahh' anymore.