The problem with any functional vent is at the end of the day, it's still just a hole in the hood that lets water in. Here lately since I've been DD'n the car, I carry my hood vent covers and an impact driver with a 10mm socket in the trunk in case it rains. That way I can just zip the covers back on real quick before sh*t starts getting wet.
Even in a pretty decent rain, you would probably be fine with the RS hood vents uncovered as long as the car was in motion. But if you were parked during a downpour, your engine bay would probably get pretty wet. For larger vents like the Evo 3 ones, I'm not even sure how you would deal with the water. I suppose just never driving in the rain would be the only real solution.