Evolved Enlarged Claws: Increases damage to isolated targets by 12% of their missing health (max 200 vs monsters). Increases range of Taste Their Fear and Kha'Zix's Basic Attacks by 50.
Evolved Spike Racks: Causes Void Spike to fire three spikes in a cone. Void Spike will consume Unseen Threat to slow and damage all enemies struck.
Evolved Wings: Increases Leap's range and causes kills and assists to refresh Leap's cooldown.
Evolved Active Camouflage: Void Assault can be cast three times. Kha'Zix takes 40% reduced damage while in stealth.
So while Q is nice to evolve, yes, its strong early/mid game but weaker late game due to team fighting.
Originally Posted by solomid guide
Evolving your skills shouldn't be a difficult decision. Your choices for evolution should be dependent on your need to get the most out of each skill to fit the game. Some games you may find that you need to evolve his Ult, while other games you may not need to evolve his W. Keep your personal play style in mind when evolving as well. If you prefer to evolve the same 3 abilities each time, then that is what you're most comfortable with, and that's what you should stick with.
Try to keep in mind what skills should be evolved before others. You may like to use the same evolutions every game, but you wont always be able to stick to the same evolution order. Increased escape/chase capability may be more important in one game, while another game may warrant an increase in ranged attacking ability.
Kha'zix receives an attack range boost of 50 units for his basic attacks and his Q. While it may not seem like much, 50 units puts Kha'zix out of the auto attack range of a lot of other melee champions. This evolution also increases his Q's damage against isolated targets by adding damage equal to 12% of the target's missing health, effectively giving him greater damage against targets who are missing large amounts of health.
This Skill will most likely always end up being one of the skills you evolve as the range and damage increase are very beneficial.
Allows Kha'zix to fire three bolts, instead of one. The bolts are fired in a cone pattern. His W will now apply the effects of his passive upon use, which will apply the passive's effects to all enemies that are struck. Minion waves and camps can be cleared much faster, and strike multiple champions that are spread apart.
Can make jungle clears go faster, the firing cone is large making it easier to aim, and the passive consumption makes it a skill to keep in high regards for evolving.
Extends Kha'zix's jump range by about 150 units more then its basic range. Whenever Kha'zix scores a kill or an assist, his E is refreshed, allowing him to use the skill again. Kha'zix can jump nearly any wall on the map now(the exceptions being the the large "peak" of top lane on blue side/bot lane purple side across from the turret, directly behind red buff, and the walls behind the first turrets in mid lane), strengthening his ganking, chase, and escape potential.
Though it increases his mobility, it's the least important skill when it comes time to evolve anything compared to other skills. However, the ganking potential of the skill greatly increases.
Allows Kha'zix to activate his ultimate 3 times, instead of 2 times, before it goes on cool down. Kha'zix also takes less damage while under the effect of the stealth as well. This gives Kha'zix increased damage through passive abuse, and the ability to survive against AoE spells much more efficiently.
An additional activation improves damage output potential, and lowered damage against you improves survivability, a skill that should be kept in high regard for evolution.
So it's really a game by game decision.. Q = faster clearing of jungle, but no slow... W = slow but smaller dmg to isolated targets..
Up to you which you'd prefer at that specific time
As a side note.. I had to laugh while looking through guides and reading this:
Originally Posted by guide
Late Game : As Kha'zix the game should not really go this long but if it does you are probably able to one shot the whole team at this point. The strategy remains the same as the mid game for teamfights and seiging.