I've seen the team of randomers beat a team of Mercs or Byz quite a few times.
Like Oberyn said, Clans hardly communicate when playing, it's more whining when they die etc... The only reason clans that stack win more is because they stick together and gank gank gank. If the whole team of randomers stuck together in a group and fought together it would be exactly the same. The problem isn't the clans stacking, it's the idiot pubbers who just disperse in different directions and get killed one by one.