it's quite a heavy nerf the one Kass is eating, undeserved though? probably not, mobility happens to be really good in this game, look at kalista for example, or Zed, both of these need to do something in order for their mobility to kick in, Kass just needs to hit R...
So it's not just that he has crazy good mobility, but that it's super easy to do if compared to pretty much any other champ, except Ezreal I guess but his cd is way longer.
Now, since these nerfs are a huge slap in the face, I think Kass should get a compensation buff somewhere, perhaps lower significantly the time between riftwalk stacks, like 10/8/6 seconds or so and instead of loosing stacks immediately you'd loose stacks one by one so you can juggle it, of course it's cd should be modified properly like 5/4/3 seconds, his mana stacks would remain the same since now you can wait out your stacks a bit better.