Only you can decide if it is worth it. Is the last nth degree of handling worth any cost to you? Also, how often will you be driving the car in such a nature that you will be noticing the last bit of grip that you picked up?
Rear adjustable swaybars function to let you change the rear roll stiffness of the car. In other words, an adjustable sway bar is one part that gives you the chance to tune the
suspension to a desired over/understeer balance. To say that if you have a phb, torque arm, and lcas puts a mustang at a perfect roll stiffness at the rear relative to the front without specifying front and rear spring rates or even brands of the parts you listed (different brands' designs are not exactly the same and thus have different geometries) is rather over-simplistic. I'm not advocating that an adjustable rear sway bar is a part that a mustang 'needs,' and the terrible design of the stock sway bar is another discussion altogether; the point is that if you've changed so many variables in your
suspension, you can't always make blanket statements that will apply to every setup.