This post claims 23 kg weight savings on the F10 M5. That is 51 lb (rounding up). The brakes on the M3 will very likley be smaller (maybe smaller diameter and less thick as well) and thus less weight savings. 40 lb seems like a fairly good estimate. Keep in mind since this is spinning weight is it basically like double the amount (compared to a savings of non rotating mass). That double factor only accounts for the increased acceleration. It does not account for improved braking, nor improved handling.