And about his kit, they really should make his Q procs on his next basic attack instead of a point and click attack... I know with smart cast you just need to to put your mouse over an enemy, but still! It would be better that way. A bit like Talon's Q

The reason why you evolve Leap is for the refresh on kills and assists. The maximum range is fine after evolution, but pre-evolution the range is just a bit too short. Tack another 50-100 range onto pre-evolved Leap and it would feel a whole lot better.

Though if I had to pick just one change, and only one, it would be to change his W. Cheaper cost and always heals on hit instead of forcing you to be in melee range (Also, evolved Q makes it so that you're out of range of the W explosion even in melee range, unless they fixed that in some recent patch).

I think everyone will agree Kha'Zix's damage is about where it needs to be, it's his playability that needs work because his W doesn't play nice with evolved Q, his W is a mana hog that sucks without evolution, it isn't logical right now to avoid evolving E either, evolving Q grants zero utility (and would almost be better adding 12% of current health in damage as opposed to missing health), and R in both states allows for too little stealth over time, and the bursts of stealth are too short.