Someone gimme a TLDR there's too many paragraphs
To summarize, after rewatching the fight between Luffy and Katakuri, I’ve found reason to believe that our current Whole Cake Island scaling is very inaccurate and based on a flawed interpretation of Katakuri's quote—which is not only unproven but, if anything, creates a serious contradiction.
Specifically, the idea that Luffy supposedly became significantly stronger and faster during their fight to the point where he reached Katakuri's level in his Base. However, this isn't supported by the fight itself. From the beginning to the end, Base Luffy never managed to fight on par with Katakuri. If anything, Katakuri consistently had the upper hand in combat—even when Luffy used Gear 2nd and Gear 3rd.
While unlocking Future Sight did help Luffy perform better—allowing him to dodge some attacks, parry one with Gear 3rd, and even land a hit—he was still getting tossed around, taking more and more damage, and remained largely on the defensive. He could barely keep up, and only with great difficulty. The only moments where Luffy was able to press Katakuri or gain the upper hand were while using Gear 4th.
With Boundman, Luffy was able to briefly overwhelm Katakuri with a barrage of attacks while Katakuri's Future Sight was inactive. However, Katakuri quickly regained his composure and reactivated his Observation Haki, once again outspeeding Luffy. Despite taking multiple hits, Katakuri remained in relatively good condition and proceeded to unleash his Awakening, which allowed him to match Boundman’s offensive power. He even managed to injure Luffy and send him flying—just as Boundman had done to him earlier. And of course, with Snakeman, it was clearly shown that both fighters were finally on equal footing. For the first time, Luffy was truly able to match Katakuri’s speed and strength.
As you can see, the idea that Base Luffy = Katakuri doesn't really match with what has transpired in the original story.
Especially when it doesn't even make sense—if Base Luffy = Katakuri, then how can Katakuri also be equal to Snakeman at the same time? While weaker than Boundman, Snakeman should still increase Luffy's physical strength beyond at least Gear 2nd, if not Gear 3rd as well. And of course, in terms of speed, it's above all other forms. Realistically, at that point, Luffy wouldn't even need to use any Gear 4th forms—just sticking with Gear 2nd should be enough to easily defeat him, considering how much of a power and speed boost that form already grants him (as seen with Blueno, who was somewhat relative to Enies Lobby Luffy, only to get overpowered and speed-blitzed by Gear 2nd).
This means either we have to assume that Snakeman’s power and speed increase is so insignificant that it barely made Luffy stronger or faster than before—which, logically speaking, doesn’t make sense—or, more likely, that Katakuri has always remained above Base, Gear 2nd, and Gear 3rd Luffy all along from start to finish.
And then there's the infamous statement about Katakuri no longer viewing Luffy as inferior to him (or that he now sees him as an equal, depending on the translation). However, if you actually read and pay attention to the context, you'll realize that the quote was never referring to strength or speed—it was about Luffy's willpower and resilience and how that allow him to outlast Katakuri's own will.
The entire Luffy vs. Katakuri fight was all about whose willpower and mindset were stronger, and I think this videos explains it better than I do: