No matter how much BM powerups, she will remain inferior to Kaido.
Kaido is the main villain of this arc, not BM.
So the best version of Kaido will be still stronger than the best version of BM, that's something that is pretty much given
That's the narrative perspective. And I don't know whether a pure powerscaler would be able to grasp it.
Which in a sense tell us that the difference between the two it's not huge but not even marginal.