Neither Zoro nor Kaku was going all out. Point is Oda made Zoro vs Kaku inconclusive, and had to use a plot device(Stussy) to put Kaku to sleep when in Luffy vs Lucci's case, he showed the big gap between them with Luffy effortlessly dodging Lucci's attacks(while sleeping in some panels) and being close to KO him with one of his attacks