Go check out ultimategarage.com to see some real billet steel adaptor brackets I have seen first hand on an M3 with porsche brakes. Probably over 10mm thick and claims to be made from high grade steel at that. As others have mentioned as well, the spacers are an awful idea. Look at any adaptor bracket out there and you will see. Plenty of adaptors space the caliper but none use a stack of washers to achieve that effect.