I agree with Skippy in that the amount of heat being moved (thinking in terms of time) hasn't changed with different flow rates, just it's spread through a (larger) volume of water.
And the whole thermostat thing - thermostats are designed to keep an engine at a specific temperature - not the lowest possible temperature. I can maybe understand this in a peltier system to reduce condensation.
If you need to restrict the flow of water through your system to allow the radiator to remove heat then I think that the radiator must be too inefficient for the heat being generated. Within sane limits.
Not a flame, just my opinion.
Kneecaps