Prime Messi or Ronaldo, plus a defender of Koulibaly's level would turn them into arguably the EPL favorite. I still think City has more depth, a better manager, and more resources, but you could certainly make a case for it. That's obviously never happening, but it's funny to think about. I do think Jose knows how to grind out the league, so that would probably be an interesting situation to watch.
Adding a defender of his caliber to any of the teams in the top 4 hunt would make a massive difference, but I don't know that it would catch them up to City. I don't know if it puts them up there with Liverpool either (when they're playing well, which they are not now). But United have problems elsewhere in the pitch as well.