This is what I figured out through theory.
Anyone welcome to confince me otherwise.

When driving in slow town traffic, the mechanical fan will work due to low airflow from the front. BUT. As soon as you start to pick up speed and wind starts to come from the front, I think the airflow will eventually start to match the airflow at the back of the mechanical fan.
Meaning, wind coming from the front will start to cancel out the suction created by the mechanical fan.
For drag racing, you will score little bit of power. Maybe 5hp.But for racing purposes I think you will waste your time and money because you will have constant high speed airflow canceling out the mechanical fan.
Anyone else
Not worth the time, money and effort.

You will maybe reduce the noise from the engine compartment.
I have noticed under high revs that these mechanical fans can make alot of noise.