Sai always had armament dragon drill is a special technique passed down. And yeah Koby with conquerors no question has more potential than sai. That doesn’t mean Koby went from fodder to the all stars to stronger than them this chapter
But ACoC isn't just a better technique. It straight up makes the user far stronger. It gives far better AP and durability. It's much more versatile. I wouldn't also say that he's definitely above all commanders now but I think he's definitely at the level whilst easily beating low commanders like Jack and so on.
Also, how strong do you think is Pizzaro? Low commander? Commander level? Because Koby would have beaten Pizzaro with his new power imo. So it's guaranted to claim Koby > Pizzaro rn.
Koby definitely has the technique, now it's the question whether Koby needs more technique or more physical training. He's basically already a young Garp though.
My ranking: Luffy > Zoro > Yamato > Koby ~ Sanji.
But by unlocking ACoC - if he even unlocked this one - he'll surpass Yamato soon. His potential is crazy.