I think Kai'Sa needs a small buff to her Q.
I think as a concept it's pretty solid, but on delivery it fails in a number of ways.
It feels like it does negligible damage while offering nothing rewarding for hitting all shots on a target. If it was intended as Kai'Sa's farming tool, it utterly fails because of how it deals damage, even with the execute bonus on Minions.
So my proposition is to give her Q the ability to apply and detonate Plasma when hitting enemy champions.
This now gives her incentive to increase her missile count while also giving her solid reward for landing as many as possible on champions. Additionally, since it will only apply Plasma to champions, it does not affect her farming power nor will it accelerate her jungle clear. This also pushes her more into the hunter arch type by giving her a little push towards assassination power.