The claims that Ive seen on Tussah silk is that it adds a bit of sheen to the finished bar and that it feels smoother on the skin. Ive only added it to 2 bars that aren't finished curing so I cant say. but some soapers swear by silk.
In a tangent from the Tussah silk thread, my memory says there was a discussion on weather any true silk product can be called "cruelty free" and someone else piped up that any type of hair protein might work as well, and then the thread kind of degenerated into cat pictures...