In actual fact the designers actually use the model before as a bench mark. It could be said that the VS is actually the product of VN VP VR. Its this kind of refinement throuh models that gives us the quality and safety we have now. The whole process could be sid to be completely trial and error. For what its worth if the S2 coil pack is workin for ya go for it.
ive been told that you can put a series 2 (vp etc) coil pack on a series one but CANNOT put a series one coli pack on a series 2!....so ive been told (a few times) weired lol