Sway bars (anti-roll bars) are the best handling value on a 2008-2010 WRX. They reduce body roll in corners without changing ride height or spring rates, which means your daily ride comfort stays essentially untouched while cornering flattens out noticeably. For a street car, they are often a smarter first suspension mod than coilovers.
Front versus rear matters for balance. A stiffer rear bar reduces understeer and makes the car rotate more willingly — the classic WRX improvement. A stiffer front bar adds stability but increases understeer. Most drivers start with the rear bar, then add the front if they want a more neutral balance. Adjustable bars with multiple end-link holes let you fine-tune stiffness without buying another bar.
Diameter is not the whole story — the bar's material, solid versus hollow construction, and mounting hardware all affect effective rate. Hollow bars save weight with similar stiffness. Make sure the kit includes upgraded bushings and brackets; the factory mounts can flex or fail with a much stiffer bar.
Pair sway bars with quality end links to remove slop from the factory links. Filter by front/rear position and adjustability to dial in your 2008-2010 WRX.
After install, drive the car for a week before judging the balance — bushings need to settle and you need seat time to feel what changed. If the rear feels too lively, soften the rear bar one hole before touching anything else; small adjustments make big differences in how a WRX rotates.