To be honest, I don't see which skills Zoro can refine at this point that would allow him to best Mihawk that wouldn't also be exempt like Shanks' if we follow that line of thinking. I mean Conqueror's haki is now Zoro's best power-up, if he keeps refining that, would him beating Mihawk with it disqualify him from the title? I suppose you could say Shanks specialises in Conqueror's haki, Zoro doesn't but they're both implementing it in their attacks whilst using swords and Zoro has no issue with stuff like airslashes.
Personally I think Mihawk's stronger at this point, I still have them close but World's Strongest Swordsman is World's Strongest Swordsman. Shanks could still be a near-equal.