Gangplank Passive

So Gangplank is a fairly underwhelming champion at the moment.  His early game isn't bad but it isn't like a lot of other top lane champions with crazy all ins at specific levels (Rivan at 1, Kha at 2, etc).  He has a cool global ult which can turn the tides of fights in other lanes, but it isn't like he can use it to heavy gank other lanes either like TF or Shen.

In other words, it isn't like GP is necessarily terrible.  He isn't strong, but he has some cool global presence, not a bad laning phase, and he's somewhat entertaining after he gets his static shiv (LIGHTNING!!!!!!).  But I think that it's pretty safe to say that his late game is pretty lackluster compared to a lot of other top laners and junglers and there really isn't that much reason to pick him.

So, I propose a small buff to GP that would allow him to scale a little better into late game, and that is to make the slow associated with his passive scale with ult level like many other passives do.

What I mean by this is that at levels 6, 11 and 16 GPs passive slow per stack should increase from 7 by a certain amount.  I was thinking perhaps 7% - 10% - 13% - 15% or something along those lines.

This would effectively give GP a little more soft control later on in the game.  It would give him a tad more utility at end game where he lacks without buffing his early game.  And it would do it in such a way that he wouldn't be doing more damage or locking anyone down, just giving him a little more sticking power.

Most champions with slows have them scale throughout the game, so this change wouldn't be unrealistic in terms of other champion abilities.