Shouldn't be a problem as long as the engine is running. Takes a lot of water to cause a problem. Performance is always down as humidity goes up, but that's another thing.
The problem may be making sure there's no drainage path to the open box when the bike's parked in the rain. Either hit it with a hose from above or let it sit in the rain and see if you have any water accumulating in the lower airbox. There's a drain hose you can check to see. Runs down from the forward left of the box to the left side of the engine front. If no water comes out you're probably fine. Or pull the filter and look.
Yes, it is the best!! we just stand there after a ride and stare at it. The mods have made such a difference, just like having a new bike. And the open air box, and the pipes, ahhhhhh, the sound is great!!!! We are simply amazed!!!
The coolest thing about the open airbox is that the bike sounds just as good to people its approaching as people behind the pipes. Especially when you close the throttle coming to a stoplight or something. And you can modulate the sound by playing with trailing throttle while you are slowing down.
For me the down side was the intake drone was hammering my ears enough that I now always wear earplugs when riding for more than about 20 minutes.
When I got the K&N kit for my m900 I used the original airbox lid cut away as far as the edges, except for the front edge where I left enough lid to go underneath the front of the tank. This required squashing the front corners of the filter element to get the lid on, but the airbox didn't get any water in it. I don't know if I'll do the same with the S4, it makes enough racket as it is above 7000, and it doesn't need to go any faster.