Kai'Sa's W as it stands is a pretty hollow ability. It applies 2 stacks of her passive (3 if evolved) and does a certain amount of damage. The problem is 2 fold.
1: Her W locks her out of being able to move, it has a 0.5 second cast time which does not seem like much.
But what ends up happening is you auto 3 times and attempt to proc your passive with W and you stop moving allowing them to get out of your AA range. If they so happen to live the passive proc, you cannot follow due to having such Low base MS allowing them to get away in most situations.
2: In most cases, having even 1 attack speed item allows you to proc more passive stacks than the if you used your W on them. Which if you are in lane you wouldn't want to use anyway to due it being blocked by minions. The exception to this is an evolved W. Which applies more stacks if it hits than if you had 2.0 attack speed.
So the issue is this. Why should you use an ability that locks you out and stops you from moving. Is nor guaranteed to hit unless you are in their face, and can be blocked by minions. Over normal auto attacks? Yes it provides vision and long range assault went it hits. But that is only if it hits. So much of the abilities potential is lost by the lock out time.
The Solution should be:
1: Either reduce the cast time of W.
Allowing her to at least stay in range of her opponent when she uses it. So reducing it from 0.5 second cast to 0.3.
2: Adding a slow to the W.
This would allow her to at least keep up with her opponent and not lose them due to her low base speed. Evolved W is in most cases is really hard and unnecessary to get in a game. AP Kai'Sa is so reliant on hitting her W that it makes even attempting to get an AP item for her useless. Its far more efficient to go Full Crit/ Attack speed or full DPS armor reduction with Black Clever/BORK, That it makes AP Kai'Sa none-viable in most situations.
Doing either one of these things would make her W far more rewarding to players.