I was using Eibach front at soft and stock rear bar for few seasons. It is definitely better set up than stock(my lap times improved)..but car was a bit understeery. I tweaked front/ rear balance over the racing season, but behavior persisted.
Few months ago I put rear bar in too( I bought full Eibach set, but was reluctant to put rear one in because of feedback that car will become too tail happy)- on softest setting. I did not get chance to test it on the track, but on highway ramps car feels more eager to rotate and I really like the feeling.
Theres probably no simple answer- it will come down to how you like your car behavior and how will it interact with other components. If you can afford full kit- try it, and if it sucks- ditch it
Correction:-went in for spring cleaning-bar was on the middle setting. Geez, not sure how I survived driving in the winter!