What Luffy needed against Kaido was AP. Something to block his club strikes while simultaneously inflicting enough damage to take him down. For both of these aCoC was the perfect PU because it's an AP amplifier.
Luffy was already able to dodge Kaido and keep up with him in the speed department pre aCoC.
That's the difference with Kizaru. When we saw them fight Oda immediately used Snakeman because it's Luffy's fastest pre G5 ability. AP wasn't the issue (well it was as well but we assume he just didn't use aCoC in Snakeman) but speed.
I know G5 doesn't visually impress enough to show it but the mode is both stronger physically and faster than any previous Gear, it combines both which is why it is his peak.
I know that Kaido keeps playing around but I'm pretty sure that when he entered drunken mode is when he truly went all out.