I finally had a day warm enough to get my Daystar hood cowl on.

 

Did you have to make any holes in the hood?

 

Lots of holes, and some chopping, too.

It looks good, but I see a potential problem. While it will let out some heat while moving slowly on the trail, on the road it may reduce cooling capacity.
The vents are right in the high pressure area in front of the windshield. Any air that enters the vents is air that won't be going through the radiator. It may never be a problem, but if the JK is being operated in a way it needs all the cooling it can get, this may tip the scale.

 

You misunderstand the function, or the direction of the hot air flow. Hot air does not enter through the vent, it is expelled (or drawn out at speed) from vent.

 

I understand the function very well. Hot air comes out during slow running on the trail. But on the road, there is a substantial high pressure area in front of the windshield, as I mentioned in my post. That pressure will cause air to enter the vents. That's one of the reasons for the rubber seals on the leading and trailing edges of the hood. They prevent air from entering the engine bay at those two higher pressure areas. Air that enters the engine bay from above is air that won't be going through the radiator.

Also, as I mentioned, it may not be a problem, until the cooling system is operating on the edge of its capacity.

A good test would be to put some little streamers on the vents, and see if they indicate airflow into or out of the vents at road speeds.

 

That is exactly why the shaker hood scoops on Trans Ams were mounted facing rearward ...

 

Exactly. And notice how it is sealed all around to prevent high pressure air from bleeding into the engine bay.

 

And to keep engine compartment air from getting into the cowl.

True cowl induction only works if all the air entering the cowl is fed to the intake. If it is not sealed up then it is not cowl induction. It is simply heat expulsion when stopped and can mess with the air cooling dynamics when moving.