I'm always skeptical when I hear about aftermarket dealer's assurances as to performance equal to or better than the original equipment. The stock rotors are spin-balanced at BMW after the OEM manufacturer (Brembo, Ate, Balo, etc.) ships them to BMW. BMW shaves metal off the rotor to achieve the proper balance. There is a year warranty if the rotors warp as a result of suspension issues. The aftermarket dealers are almost as expensive, and attach less valuable warranty terms and conditions with their products. Some manufacturers claim to spin balance their rotors, but they may use less reliable processes to achieve the proper balance. Any comments ?