when you lower a vehicle the control arm starts to become parallel instead of of angling downward from the center of the car
/ \; when this occurs the roll center is lowered, the further the roll center migrates from the center of gravity- more roll is introduced- obviously not desirable. this is the reason why one has to be very conservative w/ lowering on the Impreza
this kit (and others) space the ball joint which in turn causes the control arm to go from __ to \- this raises the roll center (keeping it closer to the center of gravity)
if you only space the ball joint, the tie rod is then misaligned and causes an increase in bump steer (not desirable), however if you space it the same as the ball joint you maintain the same geometry as the factory and no increase in bump steer
the WL kit offers both a spaced ball joint and a spaced tie rod end
Thing is.. unless you've done something to fix the rear suspension travel issue, you've got much bigger problems than bumpsteer, camber loss and roll center on the front suspension if you're dropping your '08+ Impreza that low.