I don't think there is a clear answer. ABS works by pumping the brakes very fast. It doesn't necessarily stop you faster than locked brakes. Instead it provides steering control authority while you are coming to a stop. The questions then become: does a fresh piece of tire brake better than a piece of tire that is getting hot from sliding friction? Does the act of rolling the tire reduce the effective speed of the sliding part of the friction equation (because the slower you are moving the faster you come to a stop)? I don't think both ABS and engine braking in combination would affect the stopping distance because you are already locking the wheels up as much as the ABS system allows but there might be an advantage to agressive downshifting because the tires continue rolling and that might reduce the sliding component force the wheels need to overcome more effectively than the ABS does by pumping the brakes.