It mainly improves the efficiency of the fan, as any air pulled in by the fan has to come through the radiator, thats why all manufacturers fit one of some sort.
I would mainly expect to notice it on hot days and like when off roading the engine is running faster in low gears but not travelling through the air fast.
The fronty execept perhaps the 2.8 diesel is not particularly well cooled and the radiators are barely up to the job so I would fit one myself and it is handy for keeping the fingers out of the blades
Mike