It may be worth it after all. You wouldn't need that giant heatsink, or any of the cooling fins on the engine block. That weight savings alone could equal the weight of a tiny radiator, and a small electric motor to spin the water pump. Food for thought Mr. Rossi.
Lets real in the dreaming a little bit here guys. There is no way you are going to implement watercooling AND save weight. The negligable ammount of weight you'd reduce by eliminating cooling fins would be replaced by the weight of the waterjacket and fittings easily. Then the weight of coolant lines, coolant, a pump, and a radiator is all going to be extra weight. In the end you may get more stable temperatures, but it is unlikely you'd get better overall performance since the weight increase would more than offset any power increases...