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Hey Phase! Nice to meet you. (waves)
You bring up some good points. First, the hood shroud is also for sound deadening as well as insulation. That's also what the bubble stamped on the hood underside are for; to disrupt sound waves near the engine frequencies (as well as provide structural support). So I forgot to mention that a fortuitous side effect of engine bay insulation is reduced engine noise. I actually fully sound-proofed my car Champrius (I'm a huge audiophile) and every aeromod made the road noise lessen. It's got a super quiet sound stage now. ;-)
Surprisingly, the hood insulation has minimal impact on driving in hot conditions or for a long time. What's far more important is radiator airflow. So whether you are doing grille blocking / have a engine belly pan is a much a larger concern.
There are different ways to approach the safety issue. The optimal performance design is an inlet which is 40% the radiator surface area, diverging duct to the radiator, converging duct out of the radiator, and then an outlet +15% the size of the inlet. That is safe for super high performance vehicles like racing cars, and thus is an upper bound for passenger vehicles. On my car I've been using a 30% inlet with no ducting with no problems for years, under all kinds of adverse conditions.
The way to determine safety is to get a measurement tool of some sort and monitor coolant temperatures. I use Torque Pro with an ODB II reader, but really anything will do. Start with the stock configuration and you'll quickly discover the operating temperatures. On my 2009 Toyota Prius, it's 185-190F. I can put nearly any load on it in any (hot) temperatures, and the needle barely budges. (Cold is a whole other ball game.)
Then as you make modifications (like grille blocking), do test runs under loaded conditions and note the temperatures. What should happen is they remain reasonably stable until you hit a critical point, then you'll see large temperature swings. Then just back off to the last area / design that had stable temps.
To give you a ballpark, I discovered 30% radiator area grille block was completely safe under all loads, so that's what I kept it at. 21% was very reasonable and solid except for super extreme conditions (i.e. driving in Death Valley in a black car). I'm currently running a 12% grille block test, which I can get to jump +20F just under normal driving conditions (I'm testing other stuff, so I want a super sensitive inlet).
Don't take that as canon, though. You should measure it for your vehicle, with your mods, under your expected conditions. But the proof is in the radiator pudding. If the temperatures are stable under whatever scenario you are concerned with, then that's how one actually ultimately determines safety.
By the way, older cars used to have settings for the radiator inlet by season, i.e. "Summer", "Winter", and "Spring/Fall". If you do a lot of driving in a huge variety of conditions, you might benefit from a variable inlet and outlet system. Something near 40% / your max for Death Valley, and 10% / your min for cold weather or somesuch.
Hope this helps!
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