I would think that fitting a lower temp thermostat would be the most efficient way to reduce engine temps in a D2 or install an in line thermostat instead of the standard setup 

The only time my D2a V8 warms up is when travelling at low speed/low revs on a hot day, at cruising speed 90+ K's the engine temp stays at 90-95 even when towing my boat.
At low speed a scoop would have little effect But vents may help to evacuate the hot air around the engine somewhat.
I did however notice that after fitting the snorkel my air intake temp dropped dramatically at cruising speed, I don't know if this makes that much difference But it can't be doing any harm.
There may be some cooling advantage fitting a scoop or vents on the bonnet But I think it would only be minimal and not worth the cost/effort to do it.
Cosmetically if you like that sort of thing then go for it because I can't see any problems with water ingress unless you are driving through a very heavy deluge or more critically if you are driving through deep water. 
I think that fitting a scoop/vents won't do any harm to an engine But the gains would be minimal at best.
 
			
		
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