Remember this: compromise.
You are running relatively low rate rear springs so any anti-roll bar is going to contribute more anti-roll than a higher rate spring. The more the anti-roll bar contributes to the total anti-roll of the car (sway bar plus springs) the more cross talk (read less independent suspension) you are going to have.
Given the rear motion ratio you have a lot less spring rate at the rear wheel than the front. As @
jamal has suggested a better fix would be higher rate rear springs but then there is the question of damping/valving for the new springs.