Desirable pressure/airflow/wattage for radiator fans?

For my workstation water cooling project I plan on ordering some of these 18W fans to mount to the radiator. I plan to test with PWM what output is optimal for the whole system.

I would like to see if it's really worth it to pay more for these heavy duty guys.

I figure since the bottle neck in any fan dependent cooling system is the air thermal conductivity then it should be advantageous to pass more airflow but I don't know at what point there is another limiting factor.

I want to get better metrics on this.