The game changer was ACoC its what made Luffy into a top tier, and he hasnt used it yet even in G5.
Eh he didnt seem difficult so far, just very slippery and hard to tag. Hence why Luffy went G5, to tag him.
This means Kaido> Luffy as well, because apparently G5 has a time limit.
Bear in mind Kaido fought 1010 Luffy toe to toe, then fought 1027 Luffy toe to toe then fought G5 Luffy toe to toe and even overpowered him at time. He simplay was adjusting his power level as Luffy was powering up because he enjoys a good fight. Kizaru is on a mission so its not like hes trying to have a fun fight.