The bushings that hold the sway bar to the frame are only there for that reason. Why they are rubber or other elastomer is for noise/vibe isolation only. The sway bar acts by twisting action on it from the opposite ends. The bushings are not gonna grip the sway bar tight enough to increase the twisting force (torque) in the sway bar. If the mounts are sloppy enough so that the sway bar rocks a little, that will decrease the action of the sway bar.