I don't think there's anything significantly different to Sark's genes compared to other Warriorborn. "Spidery" is Jim's usual adjective for a character that's deadly, graceful, and competent but the reader is supposed to be creeped out instead of impressed by the character. The same thing happens for the Vord queen and for Mavra.
I believe there is a specific quote that says he moves like a Silkweaver... not a Cat. So it is at least implied that there might be different styles of Warriorborn... or someone is mucking about with the designs.
The reference to "batman" is in the traditional dictionary sense of the word. "a soldier assigned to an officer as a servant."Without the reference to the cat-like eyes I'd have said the half-souled reference was more akin to the berserkers referenced in the 2nd Dresden novel. That is, people with a soul of a beast, but a human shell.
they also keep calling him her "Batman" and I know I have all kinds of imagery tied up with that one
Half-soul appears ( times in the novel, always from Rowl. Personally I think they are being called "Half-souled" because only the Cat half counts from Rowl's perspective.