You see marco doing the same against Katakuri and Smoothie? The same hes doing against King and Queen?
I think Marco is fast enough to overcome FS, the issue is I don't think he can overcome Katakuri as easily as Snakeman attained.
But Marco wields the regen hax on his own arsenal, so overall I believe he should take him out in a fight.
I believe Marco has great chances to vanquish King too 1on1, but King has shown little, so I shall lend him more benefit of the doubt, at least momentarily.