It's a combination of several things:
- Kaido hyped it in a superlative manner: "a handful of the very strongest" sounds like "a handful of Top Tiers".
- Kaido reiterated again that Luffy had acquired "a powerful weapon".
- This is the power up that made Luffy a Top Tier.
- Base Luffy went from getting low diffed by Hybrid Kaido in 2 seconds to pushing Hybrid Kaido to upper medium or lower high difficulty.
- Luffy only lost because his technique was still unrefined.
- So even with an incomplete technique, Luffy jumped 2 sub tiers because of this power.
Uber Busoshoku simply does not have the hype, portrayal or feats that Uber Haoshoku does. Luffy actually jumped 2 sub tiers after learning how to clad himself in Haoshoku. Meanwhile, learning Future Sight did not boost him anywhere near as much.
I do not claim that King has Haoshoku coating, he simply may not. Haoshoku coating is far above Future Sight as a power though.
King's current portrayal is pretty crap yeah. But if he's going to fight Zoro, then Oda will hype him up.
He may simply not fight Zoro. In that case, it's fine for him to be Katakuri level.
Katakuri being a big deal in his own crew doesn't matter?
Ulti and Page One have been far more impressive than the likes of Snack, and they are the weakest of the Tobi Roppo.
The Beast Pirates executives are simply >> The Big Mom Pirates.
Post Udon Luffy needed G4 for Ulti.
Yamato > King > Marco > Katakuri.