This is from a Whiteline FAQ:

The fitting of larger swaybars (rear and in general) has two main effects, vehicle balance in terms of understeer and oversteer, and increased roll resistance. Both of these can provide increased overall grip levels.

As most factory vehicles are biased towards understeer, fitting of the larger rear swaybar will help in providing a more neutral characteristic in the handling at the limit. This is due to the increase in roll stiffness at the rear, which loads the rear wheels more unevenly and provides slightly less grip at the rear than previous. At first this may sound sacrificial, however, as the rear end is resisting more of the roll, the front end resists less in proportion, leaving the front wheels more evenly loaded, therefore more available front end grip. In the end an increase in overall grip can be achieved by balancing the vehicle. A WRX or other front torque biased all wheel drive vehicle will benefit even more due to combined front end steering/traction demand.

Another effect of introducing larger rear bars is that the roll stiffness is increased, and chassis roll is reduced, this also reduces the effects of "roll camber". Roll camber is the variation in the wheel/tyre camber setting due to chassis roll, and during cornering usually results in the outside wheels gaining positive camber. By increasing the roll stiffness and reducing roll camber effect, the wheel/tyre stays closer to its wheel alignment setting or optimal setting. This can increase the overall cornering grip available, as the wheel/tyre does not lose as much negative camber at the limit.

The balance (and grip increase) of the car could also be achieved by reducing the front swaybar stiffness, however its roll stiffness would be reduced and roll camber would suffer. This would lead to large amounts of positive camber being gained on the outside wheels/tyres when cornering. This would result in a wheel/tyre that would not be at its optimal camber setting at the limit of handling. This could be remedied with large amounts of static camber to counter act the positive camber gain, however the resulting tyre where, and straight-line handling effects would suffer.

To maximise wet weather grip, a softer overall setting would be required from dry settings. The reason for this is that a wet track cannot give the same friction values as a dry track and therefore overall grip will never be as high as in the dry (hence the amount of chassis roll will be lower as well). The suspension can therefore be softened slightly until camber starts to be compromised due to camber roll.

Lets start first by assuming that every vehicle has a certain optimum "anti-roll" value, typically expressed in pounds or kilos. Don't ask me the ideal for the WRX as there are many formulas that have to take into account the COG (centre of gravity), RC (roll centres), the resultant roll couple and roll axis front to rear. We at Whiteline establish this optimal amount thru experience and testing though we are starting to use more computer modelling software to fast track development. With this amount in hand, the next issue is to determine where these amounts are needed and under what circumstances.

Best next to jump to the issue of cornering and how much we need where and when but its important to first identify that we need to split any given corner into at least 3 segments. Corner entry is that first phase where you initially turn in, you are generally decelerating either under brakes or on a trailing throttle. Needless to say the weight shift is moved toward the front. Mid corner is when you start aiming for the apex proper with either a neutral pitch (for-aft weight or movement mode) stance or slight power application. Corner exit is once you seriously start applying power and can be either just before the apex or after. Either way, the main steering line has been established and corner exit power is being applied to maximise forward speed. Accepting the above, its equally important to then segment handling bias according to corner position. That is, a mid-engine car with factory setup will typically understeer on "corner entry" and "middle" stages but very easily oversteer on exit if too much power is applied.

Understanding that everything is a compromise, it is useful to have a disproportionately high rear roll rate on corner entry to deliver more front roll to improve initial turn-in "bite" through the front outer. A heavier front spring like a lot of front anti-roll, will act to oppose the momentary (as little as a 1/10 sec) weight transfer required to give that "bite" which can result in more turn-in understeer. On the other hand, corner exit is all about maximising grip in longitude and latitude with throttle steering used to optimise outcome. Here, disproportionately more rear roll control acts to load the inside front (particularly useful on a fwd and WRX) as the outside rear is held more upright.

The correct total amount of "anti-roll" is inclusive of spring rate, which complicates the issue but it is still safe to assume that it is better to start with less spring rate than too much. The object is to maximise grip through adequate suspension travel at each wheel, a heavier spring rate makes it difficult for an individual wheel to react to changes in road surface under roll so contact is lost. Having said that, there is still no absolute correct number or recipe with two different racecars in any 2 car race team using largely different setups depending on the driver's preference. However, in most cases it will still function within a certain optimal pie of anti-roll force.