Originally Posted by Alpha_Onion
Anybody have an answer to my question?
I'll give you what I believe to be the tuner's answer. If you have understeer, you want to make the front suspension relatively more compliant than the rear so you get more front end grip. One way to do this is to install new sway bars with a a bigger ratio of stiffness in the rear bar to that of the front. Another way is stiffer springs on the rear and/or softer in the front. And you can play with air pressures in your tires.
And, obviously the opposite would be true for oversteer.
Also note that if the rear sway bar is (or rear springs are) too stiff, you can induce more tire spin coming off corners because the rear is not compliant enough, at least with non-LSD RWD cars (like mine). I was a bit worried about that with the H&R sway bar.
So, I don't think there is really any absolute answer to your question. It depends on what you are trying to achieve. If you don't really want to change the handling performance of your car other than to eliminate body roll, then a balanced upgrade might be the best.
One of the reasons I went with a whole system was to ensure that I had a matched set up that had been tuned by the suspension engineers. One would hope they did this through a bit of a trial and error process playing with different front and rear sway bar spring rates to see what worked best on the vehicle. I don't know if Eibach did that, but the sport system plus kit seems to be really good in my car - except the car is still is a little bit tight on the track.