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The heat transfer coefficient between water & aluminum is ~14 times the heat transfer coefficient of air & aluminum. However which is "better" is completely dependent on the respective flows and thermal capacitance of the cooling medium. The closed water system will only cool the charge air if there is a temperature differential. To keep the water system from getting instantly heat soaked itself, you need a properly sized pump, reservoir, and of course separate radiator for the water. That radiator is still dependent on the ambient air to cool it. So in the end, you are still relying on the temperature differential between the charge air and ambient air to cool the system, you just typically have a higher thermal capacitance with the water to air system before heat soak takes place, as Ash said. A big disadvantage in my opinion, is how long it takes a heat soaked water to air system to cool back down. If you have a garden hose and a tank that is easily flushed, this is not problem. But at a road course where you are driving all day, this can be an issue. Once it gets heat soaked, it stays heat soaked for a while unless you drop in a bunch of ice, which is why it works great for drag racers. Also, if your pump dies, or if you tank leaks, you are done because you have no more cooling flow. So there is an extra reliability issue as well. For the Z, I think the reliability issues, along with all the extra weight and plumbing of the system would not give you an appreciable advantage unless you were using the car almost exclusively for drag racing. For drag racing, I think a system like this has the potential to do some good things if properly implemented. :)
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