The WALK increases castor angle, so although looking the same as the OEM mount, it isn't. The S3 bush doesn't have voids in the rubber that the Golf's has, so it'll be harder, but won't give you the same benefits as the WALK. That is engineered to alter caster.
Adding castor also gives you something called dynamic camber, much better than static camber for a road car. This is of benefit because it doesn't wear the tyres unevenly when going in a straight line, yet it gives camber when the wheels are turned, thus increasing the "footprint" of the tyre when cornering, rather than scrubbing the walls.
In simple terms, it pushes the lower part of the strut forwards meaning the point of axis is forward of where the tyre touches the tamac causing a trail effect, like a chopper. If you think about turning the wheel on a chopper, it leans, developing camber when done so, but runs true once straight.
It also has the effect of pushing the inside wheel down, lessening the effect of inside wheelspin, think of a go-kart, they run loads of caster and when turning the steering wheel, it'll cock a wheel as it pushes the inside one down... not as extreme on the WALK obviously, but the effect works.
Lastly, it also means you can run the fronts in parallel, rather than toe-in as the trail effect means they'll run straight without tramlining - this is entirely personal choice though, but keeps tyre temps/wear down.
