Re: Stock non-SRT sway bar diameters
On Subarus, most people change at least the REAR anti-sway bar. For instance, on my Legacy (GT with more STI and EVO greatness), I have the 20mm JDM (Japanese Domestic Market) anti-sway bar which dramatically altered the handling of the car. The OEM American one was 16 mm. Due to liability reasons, all cars are built to understeer when pushed to their limits / in low grip. Safer than oversteering and spinning into a corner when wet. In America most Japanese cars are sold with overly-safe parts which sometimes kill the fun.
Unless you track your car, there is little benefit to stiffer FRONT sways, but you will feel the upgrade on the REAR. The place you will feel it is when cornering - but don't expect to feel it when cornering in the parking lot at your local Starbucks. You'll feel it when cornering on the highway access ramps, or any other roads with high speed and tight radius corners.