the fight sounds pretty decent, although i have some objections on luffy's massive durability increase, do all AdvCoC benefit in the same manner, or is it an MC plot armor thing.
the fact that luffy is mainly using his slowest gear against kaido, plus the fact that he commented how kaido's haki is stronger, means that physical strength and speed are kinda obsolete when it comes to AdvCoC battles.
gear 3 is an increase in physical strength and size, but physical strength is useless against kaido, so luffy is only using it for the size increase.
gear 4 boundman would be the only useful gear 4 mode, because it increases size, strength and speed, if luffy can already land gear 3 attacks on kaido then i suppose gear 4 boundman will overwhelm him.