A sway bar, or anti-roll bar, connects one wheel’s motion to the wheel opposite, to decrease body roll in turns. Larger-than-stock sway bars can help your car to corner flatter without sacrificing ride comfort.
Sway bar math is kind of interesting. The stiffness of a sway bar increases with the 4th power of the diameter of the bar. To give a more concrete example, a 22mm sway bar is 46% stiffer than a 20mm bar, and a 24mm bar is 107% stiffer -- that is, more than twice as stiff -- as a 20mm bar.
Choose your sway bar carefully. Too stiff of a sway bar can cause the inside wheel of the car to completely leave the ground during cornering, and a wheel not in contact with the pavement is not helping anything. An overly-stiff sway bar can also cause the car to behave unpredictably at the limits of grip, giving up traction suddenly and potentially causing a spin.
Many of the sway bars we sell have multiple holes to attach the end links. The stiffness calculation would apply to the hole closest to the stock one, but using the other holes with cause the bar to act more or less stiff. (Longer lever arm = less stiff. Shorter lever arm = more stiff.) You can adjust the bar action to your preferences.
Note that the stock end links will work fine with stock or nearly-stock-sized sway bars, but as you get to larger bars we highly recommend more robust end links. We do NOT recommend adjustable end links and/or end links with spherical bushings for street cars, as they tend to make a lot of noise over time. The Kartboy end links we sell use polyurethane bushings and are nice and quiet.
Keep in mind that point where the sway bar attaches to the car may also have to be reinforced. The rear sway bar mounts on many Subarus are simple flat blades that are not very rigid, and larger sway bars can severely twist and flex those mounts. Some of the sway bars we sell, including our own Mach V 22mm rear sway bar kit pictured above, include reinforcement braces for the rear sway bar mounts.
Even the highest-performance cars in the Subaru lineup are still set up to understeer at the limit, so if you are only going to upgrade one sway bar we would always suggest upgrading the rear sway bar first.