Legendary Review on RB 2pc Rotor:
Jan. 2012 - Rotor Installed
E63 Front Rotor Replacement DIY - RacingBrake
June 2017 - 5-1/2 year later
Racing Brake Rotors after 113,556miles
Why light weight two piece rotor?
Indeed. People tend to mis-understand or mix up between "Sprung" and "Unsprung" weight, here is a good illustration.
Unsprung weight is it a big deal ?
In a ground vehicle with a suspension, the unsprung mass (or the unsprung weight) is the mass of the suspension, wheels or tracks
(as applicable), and other components directly connected to them,
rather than supported by the suspension (the mass of the body and other
components supported by the suspension is the sprung mass). Unsprung mass includes the mass of components such as the wheel axles, wheel bearings, wheel hubs, tires, and a portion of the weight of driveshafts, springs, shock absorbers, and suspension links. If the vehicle's brakes are mounted outboard (i.e., within the wheel), their mass (weight) is also considered part of the unsprung mass. Effects of unsprung mass...
High unsprung mass also exacerbates wheel control issues under hard
acceleration or braking. If the vehicle does not have adequate wheel
location in the vertical plane (such as a rear-wheel drive car with Hotchkiss drive, a live axle supported by simple leaf springs),
vertical forces exerted by acceleration or hard braking combined with
high unsprung mass can lead to severe wheel hop, compromising traction
and steering control. ...