In F1 it isn´t so much about the singular devices or gadgets and gizmos attached to the cars... it is more about the package as a whole and how each little trick aero device works with the rest of the car. It is not as simple as bolting on a monkey seat or f duct if it does not fit in well with the general aero philosophy of the car devised at the initial design stage. Teams that use the monkey seat will have had it in mind in the design process, so to answer your question yes it is worth it because it was not any "trouble" to "fit" it. As far as weight is concerned, I doubt it weighs too much and may even fit in with how they want to distribute the excess weight after the mandatory front to rear weight distribution mandated by the FIA. I can`not give you figures for how much downforce it will achieve. Perhaps ask McLaren or Sauber? :P