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Well the aero drag for any of these solutions is the "sinking drag" which is equal to d(mV)/dt
mass (m in slugs) is the mass of air you're bringing up to speed of car (V - ft/sec) either passively or using the fan .... so you have the area (of your grill or opening) you have your speed (mph) and you have the density of the air P/RT (slug / ft^3) ... do the calc and you get in units slugs ft/sec^2 which is lbf ... that's your sinking drag required to cool engine.
So you block off the grill and let the fan suck it in (not too efficient at sucking these fans not designed for that) but it still wants the same heat transfer (mass flow) to cool the engine... but now instead of doing it passively you want to use electricity (juice) to get the job done. Not much sense in my opinion... I agree... take the aero hit instead.
The problem is not when you're cruising down the highway letting the good times roll though... the problem is when you're stuck in traffic in 100 deg heat going nowhere fast and the fan can't suck/push enough mass through the heat exchanger.
A variable area grill block seems like a good idea albeit an engineering problem... Personally I've got a permanent full upper grill block on (those are just for style mostly anyway) No discernible negative effect so far... and this winter I hope to see what a partial lower block can do...
This is one aspect the EV's definitely have going for them.
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