Hey Andrej,
shadow models make the game faster, of course. When your character has 14k the shadows would need another 14k too, but if you have a shadow model with lets say 1k you have an overall count of 15k not 28k.
Makes a difference, right?
The backface culling makes it also faster, as only polys were rendered which can be "seen".
Sorry, but I have no answer for the stencil shadow question.
regards
Amo