Not scientific but just intuitive thought; I imagine the "ram effect" of a 'scoop' (snorkel) would overcome the mild pressure differences (gradient) between above and below the engine bay, provided the vehicle's speed is sufficient to induce a ram of air through it. Not that it matters but my guess is that was it's intended design initially; for track use when the car is constantly moving at fairly high speeds, to "force" (ram) cool air into the engine's intake area.
But that really wasn't my earlier point. My thoughts were more about a lack of air flow at slow speeds or standing still, with heat soak building up in/around the IC. The same is true for the pressure gradient (above vs below pressures) without a snorkel...there is essentially zero air movement when you are not moving enough. So with or without a snorkel, I think a fan is best. It will move air (in either direction, based on the direction of the fan) across the IC when the car isn't moving fast enough for 'natural' air flow. And it will enhance the air flow (up or down, with or without snorkel, depending on the setup) when moving...most fans should provide more CFM than any 'natural' movement/flow.