Two month later update. I was worried they'd look a little cheesy, but I decided to give the GNX fender vents a try.
Did some driving around today, and after getting up to operating temperature, I used an IR thermometer on the fenders. The vents themselves were consistently 10-15 degrees hotter than the fenders immediately above and below them. Also, sitting at idle, you can actually feel warm air coming out of them.
While detailed underhood airflow info is going to be pretty much impossible to come by, I did also use the thermometer to check temps right after I parked the truck. The battery on the right, and the ECM/fusebox on the left were below 100 degrees, while closer to the center temps were much higher, as expected, and I think the alternator temps were not far off from where they had been before. My uneducated guess then, is while the car is sitting still, warm air will naturally just rise out the vents and be replaced by cooler air from under the truck. While in motion, I am guessing cooler air is getting pulled in from either the front lower corners of the truck, or around the headlights, and running over the the inner fenders, cooling everything near them, and then exiting via the vents.
If I want to get really scientific, I guess I could tape the vents closed, run the car over a specified route, take a whole bunch of underhood measurements, let it cool down, then do the same with the vents opened up. Not sure how I'd correct for environmental variables, though.
For now, I am pretty happy that there is a positive effect. I am still going to keep an eye out for some factory-style hood vents in the junkyard, for additional cooling near the center of the engine compartment, but it is not a high priority.
