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The radiator has no influance in how fast the engine warms up; the thermostat is closed until the coolant reaches ~180 (i know each engine is different, but usually 160-190F). Coolant will not reach the radiator until the thermostat opens (when the engine is warm)
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that's true, but that means that all air that passes trought the radiator up untill the point where the thermostat opens is 100% useless drag, and even after the radiator is used for cooling for, perhaps a few minutes it might be able to cope without direct airflow.
the fact a lot of people employ full grillblocks proves that the average cooling system is able to cope with engine cooling a large portion of the time WITHOUT a lot of air supplied direcly to the radiator. however the rest of the time where cooling air is reqired (and most grillblockers manually intervene) the average joe would just blow up his engine, unless the car is smart enough to remove the blockage itself... since it can swith on a fan why not have it open the grill just prior to that.
if the cooling is responsible for 30% of all aero drag eliminating this even 10% of the time (but most likely much more) would mean a 3% improvement in average overall drag) that's going from a 0.29Cd to 0.28, but since the system would likely open most on stop go traffic where aero is of little improtance the benefit would be far greater!
as far as air venting trough the bonnet, it's a good idea but pressure differences are indeed imprtant, there's negative pressure at the front (lift) where the air moves faster to make it over the car, and positive pressure (downforce) at the back where the air hits the windshield, and where the cooling inlets are located as this pressure forces air in (also the reason why the radiator is in front of the car as there's also high pressure)
good airflow needs a pressure difference... so the bigger the difference between the intake and the oultet, the better air will flow
so, the less air you need to get the same cooling and so the smaller the intake can be = less drag