The difference between various brands is in the amount of energy loss which it reduces. The more energy lost, the less energy is transferred to the pack. But the speed of the cue is not a measure of such energy lost, and either is the speed of the cue ball.
the velocity of the cue ball is a good indication of how the cue transfers energy, a cue that is more efficient will send the cue ball faster than the less efficient cue assuming they are stroked at the same speed. after the cue ball is hit, (assuming a controlled hit on the cue ball) the only factors that affect how much energy is sent to the rack are the velocity and mass of the cue ball, and because the mass is constant, the velocity is the only thing that affects the rack (again, assuming you hit the cue ball the exact same way, just with different cues). of course this is over simplified, there are other factors to consider like how easy it is to accurately hit the cue ball, how much the cue tends to jump the cue ball, and spin on the cue ball, but i think these factors are more due to the stroke, tip, and shaft rather than the weight of the cue.