This looks like a great alternative to a hood scoop. How do these stand up in wet weather? Do the tires send a lot of rain through those vents?

You will get more water into the engine compartment. No problem in most circumstances. Heat rises so the open hood scoop is one of the best ways to vent the air out. If you are wheeling in a lot of rain you would usually not have as much overheating problems. Anything you can do the vent out the hot air is going to help keep the rig running cooler.