The rear beam does not really change camber when lowered, if it does, it is marginally different. The shims would go behind the axle stubs that mount to the beam. The problem is that the brake carriers also mount on the beam, not the axle stubs. I have heard of 1-2 people who put shims in and they did not report brake issues, but I have not seen any long term followup. I would worry about the caliper or piston binding due to the different angle. Eventually the pad would wear, but I see the potential for binding to be more of an issue. I don't know how much the piston angle could be off before it would bind.