Lee Sin is not overpowered, he's also pretty average as far as fighters go. His strength comes from the fact that he has a very long range engage (based on a skillshot), a separate mobility ability apart from his engage (kinda like Fizz), and good jungle sustainability. In terms of actual team fighting and duels against opponents, he's on par with other fighters like Irelia and Jax.
As far as "being on his level goes", that was not the point I was trying to make at all, but rather that in regards to Bonus AD ratios on AD caster type champions, Zeds is not especially high, even slightly lower than average.
Trade off map pressure? I don't know what you mean by that. If you're talking about split pushing, every AD focused champion with some mobility is better at that than champions that don't have that combination of traits(champs like Rengar and Master Yi are notorious for this).
Most importantly to the topic at hand, his numbers are not very high, only around average.
I feel like, you're just kind of emoting because his damage feels really high (since he has multipliers around his ult, passive and yes, his AD steroid), but he needs to fulfill a lot of conditions to get all of that damage off in proper order - so when the stars align and Zed plays properly, his damage becomes really big, but it's big because it's a sum of a lot of parts, not all just straight unconditional casting power like an Annie or Talon (which is just no hoops tons of DAMAGE RIGHT NOW). If you look at his damages compared to other champions, its really not that extreme - especially since he's a caster assassin with no real CC or utility.