OK here are my thoughts and if any on disagrees tell me why.
1) GPM (Gallons Per Minute) is what really matters when you talk water flow. If you can move 10 GPM with either a 3/8, 1/2, or 3 inch hose you are still only moving 10 GPM. The only thing that changes is the volume of water moved. And this is not always a good thing.
2) Moving as much water as possible is good IF you can COOL that given amount.
3) Heat transfer from the water in the radiator is what you want to focus on the most. The water needs to stay and pass through the radiator for a given amount time to allow as much heat to be removed from the water before going back out to cool the hardware. If the water passes through the radiator too fast the cooling action does not have sufficient time to remove the unwanted heat. If the water takes to long to flow then the hardware will overheat. Allowing the water to stay in the radiator longer (but not too long) can mean better cooling.
4) You can only get the water as cool as the ambient temperature -if you are lucky. If you have everything spec'ed out right and have the perfect setup you will end up being a few degrees above the surrounding air temp. If you cool the air before it passes through the radiator and the air is below the ambient temp then you are of course going to get much lower water temps.
5) What is needed is a thermostat to control the water flow which is controlled by the CPU temp range that you want to will keep it within that range. Just like a car, the thermostat helps keep the engine cool and at the optimal temperature. If the engine gets to hot then it opens and allows more cool water to pass.
Just my two cent on the subject :-)