Your theories sound legit, but I think the cons outweigh them. I'm sure the engineers who designed the jeep took those theories into consideration. The hood vent helps only to a certain point. At speed, some of that air rushing underneath will be diverted upwards only because heat rises. This will not cool off the engines as they were designed to operate at 210°. If the engineers thought that hood vents would cool off the engine compartment, they would have incorporated vents into the original design. I think both kinds of vents are just a cool factor.
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