I don't think Hiryu Kaen would've killed Kaido, but it doesn't really make sense to think that Ashura was actually a full strength attack considering Zoro was essentially dying by that point. It's interesting that the only times Oda has ever used Zoro as a hype tool for Luffy (vs. Arlong, vs. Kaido), he made sure that Zoro was in terrible condition.
Besides, I don't really know why Oda would let all of the other 4 supernovas on the roof get their attacks in on Kaido, but when Zoro goes in with Hiryu Kaen, he makes it miss. The only logical explanation for that is that Hiryu Kaen would have done more damage to Kaido in that moment than what Oda was okay with, but exactly how much damage remains a mystery
Besides, I don't really know why Oda would let all of the other 4 supernovas on the roof get their attacks in on Kaido, but when Zoro goes in with Hiryu Kaen, he makes it miss. The only logical explanation for that is that Hiryu Kaen would have done more damage to Kaido in that moment than what Oda was okay with, but exactly how much damage remains a mystery
but at least on the rooftop, I doubt it was stronger than asura