A sway bar is more accurately described as an anti-sway bar in that it's attempting to reduce sway, or lean. It does so by coupling the suspension on either left/right side to the other through this bar.
Generally up to a given point and depending on wheel base and center of gravity, the more you resist lean, the less total grip you can generate, but the quicker you get to that maximum weight transfer and thus maximum grip in a corner. When you go with more lean, you actually generate more grip because you transfer weight better to the outside wheels which are doing a lot more of the cornering than the inside wheels. But now transitions between left and right become more dramatic, and a bit of a pendulum effect can be utilized (think of the "Scandanavian flick").
The ZOK bars are stiffer than the FE3 bars, and thus resist roll more. When you increase the roll stiffness in the rear compared to the front, it tends towards oversteer, and when you increase the the front roll stiffness compared to the rear, you will get more understeer.
Getting the ZOK rear bar alone will be quite noticeable. It will be far more neutral, and depending on how much power you have, and what tires you run, perhaps even more prone to oversteer. If you get both front and rear ZOK bars, it'll feel a lot more like stock in terms of under/oversteer, but will not lean as much under higher cornering loads. So if all you want is mostly less lean in a corner, get both front and rear. But if you want it to be more neutral, and understeer less, go with just the rear bar.
It used to bug me that my car would understeer on tighter on ramps, and that the only way to get it more neutral was to break traction under power. Now the same on ramps have me generating four wheel drifts on command. Albeit, the rear ZOK bar was the last suspension addition after many of the other frame stiffeners. Don't forget all the other practically required body stiffeners like a backbone, a probeam, and even a LVFCB.
