Feral disco I am part way through engine bay ventilation upgrade to my V8 D1 so maybe my humble opinions can provide some guidance for you.

First things to remember is get this wrong and you can make things far worse under the bonnet and for air to move it needs a pressure differential. This subject has nothing do with the car movement.

The stock Disco relies on the pressure difference across the core of the radiator caused by the higher outside pressure and lower pressure inside the engine bay being maintained by pressure escaping through openings at bottom of engine bay.

I don't have any measuring equipment to prove this but I am hoping my solid steering guard protector helps to reduce the pressure build up in the engine bay. My thinking is the guard is reducing a pressure build up from turbulence particular in the area just behind the radiator.

The next target is to get more pressure out of the engine bay. There are only two places to do this. The bonnet and the two sides/guards.

A Bonnet Vent needs to be letting pressure out of the engine bay not raising the pressure by forcing air into it!!!!

I am guessing the pressure above my bonnet is lowest at the front and highest at the rear or near windscreen. That is why air conditioning air vents are located at bottom of screens where outside pressure is at its highest. Under the bonnet I also think the pressure will increase from front to back, particularly if my steering guard is doing its cooling job.

That says to me the pressure differential between above and below the bonnet increases towards the front of the bonnet. Put a vent anywhere near the rear or windsceen or where outside pressure is at its highest, particularly a rear mounted reverse facing vent, then there is real danger of the flow going in not out of the engine bay. That increase in engine bay pressure could suddenly do horrible things to radiator efficiency.

I am currently in the process sourcing an appropriate reverse facing vent and locating on the bonnet centreline as far forward as I can or may even test out a GU style forward facing number. I have a blacked out bonnet so what ever the outcome should look OK.

That done I might then turn my attention to side vents to further improve flow out of the engine bay.

Hope this helps or at least widens the debate.