Combined with the 12" rotors, it stops much better than stock. And the f-body calipers weigh a lot less than the stock calipers, which offsets some of the weight of the 12" rotor. [/QB]
The weight claim is misleading though. Your rotor weight is what is important with stopping. The more rotating mass you have on an axle (bigger/heavier rims, bigger/heavier rotors, etc), the longer it takes to stop.
Heavier calipers won't effect your stopping distance, just your speed.
I'm sure the larger pads and stuff outweigh the heavier rotors...but the the weight savings on the calipers won't help you stop quicker.