Single Rotation (Passive, Q, W, E):
846 Base Damage
390% Bonus AD
100% Total AD
Full Combo (E, Passive, Q, W, Q)
1101 Base Damage
600% Bonus AD
100% Total AD
Single Rotation (Q,Q, E, R)
1140 Base Damage
480% Bonus AD
8% Target's Missing Health
Full Combo (Q, AA, E, R, Q)
1140 Base Damage
480% Bonus AD
100% Total AD
8% Target's Missing Health
Looking objectively, Lee Sin does have a bigger burst than Kha'zix in a single rotation, however, during a full combo, Kha actually out damages Lee due to the 120% Bonus AD advantage, which outweights the 8% missing health by a fairly big margin (With a 400 Total AD Build, the target would need to be missing 4380 health for those 8% represent the same amount as those 120%).
If we take in consideration that Lee's damage is extremely gated by his ult, which has a 60 seconds CD and that Kha'Zix combo is gated by his E, which has a 12 seconds CD AND resets on kill, in the worst case scenario, Kha will be able to do his combo 5 times for every 1 time Lee pulls his combo off.
Without his ult, Lee's damage falls to:
540 Base Damage
280% Bonus AD
8% of targets missing health
Another point to look into, is that Kha'Zix Q, his main source of damage, is in a 4 seconds CD, with a potential 60% CD refund, reducing it to 1,6. Lee's Q, his main non-ult source of damage, is gated by a 7 seconds CD.
One extra point to take in consideration, is that Lee is gated by energy and doesn't have enough energy to use all his abilities without auto attacking in between to restore it, extending his burst window considerably. Kha has enough mana to use his whole combo once without any worries.
In short, Kha'Zix, in comparassion to Lee Sin, is more fit to assassination due to having 5 times as many windows of oportunity as Lee (being gated by his E [12 secs], rather than an Ult [60 secs]).
However, they both have basicly the same win rate, the same learning curve and the same winrate-by-game-lenght pattern. What does this shows? That you don't compare a fighter to an assassin because they fufill different roles using different gameplay patterns.