As Quadcammer said, the major reason to add rear adj control arms is to get additional negative camber in the rear. Most Spec Boxster's can get sufficient rear camber on the stock arms so there may not be much advantage in adding those.
An adjustable rear sway bar adds another way to tune the handling. Typically, engineers recommend adjusting the end of the car that is causing the problem. For example, if you have understeer, loosen up the front end. If you have oversteer, tighten up the rear end. If you run out of adjustability at one end of the car, then go to the other end of the car to move the dial further in the preferred direction.
In terms of adjustability, generally the car should be mostly neutral but can lean towards understeer or oversteer depending on your comfort and skill.
If the handling is mostly neutral and the tires are wearing evenly and have good temps across the width of the tire, then your setup is probably pretty good.
The key is to find a setup that creates handling characteristics that complement your driving style and makes you feel confident in the car. Sometimes you have to adjust your driving technique to better match the inherent characteristics of the car and sometimes the car can be adjusted to better match your individual driving style. This is the art of car setup.
To answer your question directly, no I don't need rear adj lower control arms to get enough rear camber and yes, I use the Tarrett adj rear sway bar to help balance the handling to my preference.
Beyond that, the car should be at minimum weight and the engine should be dyno tested to make sure that it has the expected power.
After that, concentrate your efforts on the person behind the wheel.
I say that because once the car is pretty good, then the driver becomes (by far) the biggest variable. And this is why I focus a lot of my energy, time, and money on improving my driving skills. In my case, I know that I am the weakest link!
