Knockback of a fainted opponent isn't everything, Luffy sent Wapol flying to another island in base 900 chapters ago, knockback counts a bit more if it is DURING the fight and the knockback itself is what is overwhelming the opponent, e.g. Luffy vs Doflamingo.
Kat got "destroyed" because G4 is an extremely strong limit break technique, he was still hyped as undefeatable, where King and Queen were not.
Sanji doesn't have better stamina considering he just fainted after a shorter fight than Luffy vs Kat, and his armor/regen was partially fake hype considering winch queen can bust through it.
Kat can casually make multiple superior to G3 level attacks with insane hardness sprout from anywhere and see ahead of time where you will be to perfectly hit you. Sanji's armor isn't at the level of blunt force immunity like Luffy so he will struggle with this.
Sanji is extremely strong and above King, but no way is he above Katakuri.