It could be that the weight penalty is an easy calculation. Stopping distance at fixed brake energy is a function of weight. On B738 you increase the ELW and so increase the required LDR with manual spd bks. On takeoff I suspect that stopping distance was calculated/certified using manual spd bks. B737's the spd bks deploy with Thrust Revs, but you are supposed to do them manually. can't remember B5757/767; too long ago.