Also, last thing on this power scaling wise. Luffy lasted half a day vs. Katakuri without CoA powerup. Obviously base Luffy can beat Ulti 1 v. 1, but it'll be a high diff fight, meaning he will waste a lot of HP/Haki and be exhausted for the Kaido fight. Look at what Zoro said:
I'd say it's something like:
Base Luffy vs. Ulti = high diff fight
G2/G3 Luffy vs. Ulti = mid diff fight
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Gear 4 Luffy vs. Ulti = no diff fight
It's clear than
anything more than a low diff is unacceptable for Luffy. So it's not just a matter of beating Ulti. It's a matter of beating her without taking a lot of damage and wasting HP, which he can't do without Gear 4. Even G2/G3 it will be a mid diff fight.