Well David Silva who was for many years a "number 10" has been able to play in a midfield three really well for Man City; Ross Barkley (sorry, let me just spit real quick) looks a better player in that role than he ever did playing in behind or from the left, Bernardo SIlva, the list goes on. There's no reason why Gylfi can't play there too perhaps the most advanced of the midfield three. The holding anchor or Regista permits a little more freedom to the two centre mids ahead of him and Sigurdsson does appear to have an engine. Good players only play well when they have the ball; this system would give Glyffi exactly that.