It's not even 0.1C better lol. It's better than plastic because it's far stronger. The main purpose of water cooling is to use water to transmit heat, not air. Water is by far a much better heat conductor than air, that's why aluminium plate will not help the heat dissipation. Plastic is not OK actually, that's why manufacturers don't use cheap plastic. I've lost touched with water cooling, they use other types of plastic that doesn't crack easily.