Kha'Zix doesn't need buffs, he and Rengar are failed designs. They either have the burst to do their job (100:0) and are broken, or they don't, and they're a liability. There's zero middle ground and I'll exlain why.
If Kha'Zix can innately burst down targets, his E will reset and he'll always go in, kill a squishy then reset to E out. So, obviously, that isn't too healthy. So, he obviously can't be allowed to do that off the bat. So what does he look like if he can't do that?
Well, to put it in a single word, he looks like a liability. He goes in, can't 100:0 the target and gets killed from his squishiness. His only escape requires he kill a target, and his ultimate is easily revealed by a sweeper.