Link to the US fix it kit:
http://www.fatcatmotorsports.com/FCM_M3W.htmThe other thing with fitting spacers is if the spacer were to fail, you would not be covered by your insurance, unless you have told them of this modification and an insurance company would be mad to insure a car with an "un-tested" safety critical component fitted to it.
I think that the spring rate would not really affect the bump steer, as the problem is purely caused by the error in the geometry, it may feel less violent with a softer spring, but the level of movement would remain basically the same, unless the spring was sufficiently hard lo limit the suspension movement.
