I have had no problems driving the car in heavy rain, even following another car that was throwing up a constant spray. The back side of the filter stays reasonably dry and can still pull air.
I did have concerns on the track once. Ran in a session right after the club racers, where several lost their engines and dumped oil all over the track. The track put quick dry ( kitty litter) all over the track right before our session and I picked up a snoot-full of quick dry all over the forward face of the filter. Didn't appear to bother the engine, but I quickly replaced the K&N filter when I got home ( $45 I think). the back side of the filter was not caked, but I was left with half the filter area available. It's a pretty big filter though.
It's a nice cold air and ram air location, but it is exposed so you do have to check it after sessions. If you thump it with your fingers, you can knock most of the loose stuff right off it.
2011 e92 M3 Monte Carlo Blue ESS 535
2010 Honda FIT
2006 SLK 55 AMG