This is thermodynamic hokum. The physics of heat transfer simply do not support this in any way, shape, or form.
Now, there are more factors than just heat transfer at play, turbulence, cavitation, heat pockets in the block, etc and those can change how some of that stuff works.
But just saying that the water is going too fast through the radiator to cool? Nope. Heat transfer occurs at the molecular level, and molecules transfer energy a heck of a lot faster than the water in your radiator flows. And while time is a factor in the amount of cooling that occurs, faster coolant speeds also means more “trips” through the radiator. So what cooling may be lost on a single pass is compensated by making more passes in a given time.
Not discounting your experience, but, your changes in cooling were 100% caused by something other than just coolant speed.
Yes, the factory used a .95:1 ratio with the high volume water pump and a 1.3:1 ratio with the standard water pump.
This likely had everything to do with fan speed and moving more air than it did with the water pump, which is why the overdriven systems used a lower output water pump. You can definitely spin the water pump too fast, which can cause a loss of efficiency from turbulence or even cavitation in extreme circumstances. The same is actually true of the fan blades as well, they’re designed to spin at certain RPM’s and are most effective within a certain range. So, keep that in mind when picking your components. The factory was balancing fan speed and water pump speed for best effect in both air and water flow, and multiple components were changed on the standard cars vs the AC cars. The whole of the system is what has to be considered, just overdriving the pump/fan may not have the best results unless you have a pump and fan that work best at that higher driven speed.