Either Rek'Sai's New Ult is Garbage or Lee Sin's Q is Godlike
Here is a realistic breakdown of the difference between the two. Considering how snowbally games are nowadays, I compared a reasonable scenario of these two junglers going in on a squishy target at half health that they tagged when all champs were at lvl 9.
Five points in Lee’s Q (Range 1100) (CD 7s)
50/80/110/140/**170 **(+90% bonus AD)
50/80/110/140/170 (+90% bonus AD) (+8% target’s missing health)
170 dmg + 54 dmg + 170 dmg + 54 dmg + 44 dmg = _492 total dmg_
One point in Rek’Sai R (Range 1500) (CD 100s)
100/250/400 (+140% Bonus AD) (+ 20/25/30% of their missing health)
100 dmg + 84 dmg + 110 dmg = 294 total dmg
Rek’Sai’s ult does 60% of the damage that Lee’s Q does mid game while being on a 1400% longer cooldown!!!
If Riot thinks this is a reasonable ultimate ability, that just shows you how strong Lee Sin's Q really is.
Now let's say that Rek'Sai tagged the squishy target with her Q, Prey Seeker:
Five points in Rek'Sai's Q
60/90/120/150/**180 **(+70% AP) (+40% bonus AD)
One point in Rek’Sai R (Range 1500) (CD 100s)
100/250/400 (+140% Bonus AD) (+ 20/25/30% of their missing health)
180 dmg + 24 dmg + 100 dmg + 84 dmg + 110 dmg = 498 total dmg
This means that Lee Sin's Q does 99% of the damage that Rek'Sai's Q and R do!!! WTF!!!!
your test is using a bad comparison.
Q can do over 300 damage with DFT and a doran's ring at rank 5, and doesn't take much ap to get it equal to lee sin's Q, which takes two casts to reach that damage. That doesn't mean it is overpowered, just that as a main ability at max rank it will do high amounts of damage...