Alright, a few things.
1.) No Kha'Zix main who watched Season 5 Worlds wanted that W buff. Walking out of the jungle to nail someone with a skillshot and let the laner lay down damage for the kill is so far from the spirit of the champion that it's borderline insulting.
2.) Kha'Zix is okay right now. He's not trash. He's not top tier. If anything, he's a little weak in most ranks. Something to remember is that he usually relies on isolation to take you down and his Q with iso isn't just "higher than average assassin damage". Isolation should really be looked at as an equalizer when compared to other assassins, because non evo Q is a joke.
3.) His diamond+ winrate at this moment doesn't matter. 52%? That's scary? He was 49% last week. That's why you don't put all your faith in winrates when looking at 0.8% of the playerbase. Shifts happen and can be drastic. Riot knows this and that's why the main rank that champions are balanced around is Plat+ and not Diamond+. Also keep in mind that the gap between the top tier junglers and the "good" junglers is at one of its greatest extremes in a while. Graves and the like are banned every game, so passable junglers are going to see play. That's why he has an 8% playrate or whatever. Teemo has like a 10% with a winrate that dances between 48% and 52%. Yet no one seems to care. Is that because he doesn't one shot you?
4.) Calling Kha'Zix flashy is a joke. There's nothing flashy about deleting someone from stealth. He's not Yasuo, Lee Sin, or Zed. He doesn't make "big plays". Most Kha'Zix montages consist of a fed as hell Kha'Zix one shotting people and running them down while outnumbered. Do you know why there are no badass turnarounds while behind in the montages? Because it doesn't happen. Yasuo can dance around and outplay for 6 years. Vayne can do the same. They can make flashy plays and turnarounds. If he's behind, Kha'Zix just dies. Don't judge what's impressive and flashy based on what a dude with 25 kills at 20 minutes is doing. Think about this; the "inSec" of Kha'Zix is the double jump, a move that does nothing but sacrifice range to look cool. Imagine is Yasuo and Lee's "big play" moves were detramental.
5.) Yeah, he's gonna be too strong because of that questionable W buff. He isn't going to be flashy though, he's just going to be lame. W will phase out R for the vast majority of players and, in many cases, will be your first evo. If you don't believe me then take a look firsthand. W first evo works on live. Try it out and then you might understand what this W buff will do.
6.) Passive buff is fine. Again, how with Graves, Gragas, Evelynn, Kayn, Kindred, and Lee running around in their current state anyone has the time to stop and say "Kha'Zix?!?!?!?" is beyond me. Kayn and Evelynn do the same job as Kha'Zix while having some pretty clear advantages. Difference? Sometimes they actually get banned. When is the last time you actually saw a Kha'Zix ban? I see one once out of every 30-40 games, and it's almost always someone I just played against or had in my last lobby pre-dodge.
7.) He's not easy to win with. Keep your eyes on that word, "win". Yeah, he's easy to get fed with, just like any assassin. Unlike other assassins, his counterplay is pulled off so easy that you can do it by accident. He's destroyed by anyone getting remotely tanky and stacking on top of each other. He's demolished by stall comps that keep the game going until everyone is full build (unless he runs Harvest, and even then, stall comps can ruin his day).
As someone who plays the champion exclusively, my personal dream is a Season 3/4 revert. That's not happening, so I'd settle for that passive buff, the non isolated W slow getting a small buff, and a slight R rework to make it a standard and competitive evolution when matched with a buffed W.
60% and 90% slows? Totally ridiculous. Whoever wrote those number up needs to sit through every Kha'Zix game in Season 5 Worlds, because I seriously doubt they have any experience with seeing those numbers on live.