Do I need adjustability in a torque arm?
...how much of a difference does it make?
...my LCA's are non adjustable, so does that mean I'm stuck at the stock angle anyway?
Not sure how much of a difference it makes. Some people swear by it. It can change how much stress your u-joints see (for better or worse depending on where you go with it). That in itself can free up a little hp.
The pinion angle is changed by the torque arm whether the lca's are adjustable or not. Even if you had adjustable lca's, they would just slide the torque arm in the bushing, they wouldn't change the angle. (unless the lca's themselves were at an extreme angle)

