I am sure that the auto makers have investigated this one to death. I would look at a corvette and see how it is done. Making 500+ hp requires an optimal cooling system design and will not be a compromise.
Ducting the fan is probably the biggest factor next to flow capacity. With tight ducting around the fan and radiator, you generate a better pressure differential wich increases flow across the rad. If the fan is in front of the rad, it should be ducted on both sides of the fan-a large area tapering, ducted to the fan then ducted to the rad. If the fan is behind the rad, you still want to duct air to the front of the rad and duct the rad to the fan.
It is a little easier to seal(IMHO) doing it this way (fan behind the rad).
I would like to say that you want to get the largest area in front of the rad ducted to the rad to get the most air forced across the rad. The other significant point i want to make is to get the air coming out of the rad, out of the engine bay. You need the air to flow out of the engine bay easily other wise you will not get flow because of air pressure in the engine bay. We want the engine bay to be a negative or lower pressure than the front of the radiator. Although additional ducts can help (IF in the right location), most of the outflow of air from the engine bay is UNDER the car. By restricting air flow under the car at the front of the car, you will create a better lower pressure behind the radiator which is what you want.
one thing that can help is sealing the top of the rad to the hood/bonnet and adding a flexible air deflector directly under the rad as wide as possible to help generate a low pressure area behind the rad. Basically, you are trying to stop air from flowing around the rad.
If you add vents, the correct place to have them is immediately behind the front wheels or immediatly behind the rad in the hood/bonnet. you do not want vents back near the windsheild, this is a higher pressure area. Opening up the inner fender wells helps air flow out through the fenders. there is a low pressure area at the front wheels.
Of course, most of this is moot when you are standing still at a red light (except the fan ducting).
LOL
Rob