save some $$ and get the AEM. The mazdaspeed has a (california air resourses board or something) CARB #, which does nothing for anyone outside of California. MS is the AEM with a splash shield. The Mazdaspeed R&D knew that the filter gets wet even during light rain (my 1st hand experience as well) because the filter is very close to the wheel well ducting vents-- so they added a splash shield.
as far as I know, the intake/engine isn't covered by mazda if your MAF or MAF wires F up or if you hydrolock. I think the warranty on the intake is when the mounting bracket breaks off due to engine movement.
I've had no problems with the F2 (filter sits next to the horn), but they're relatively hard to get and those that have em like em a lot. If you're VERY concerned about the rain, purchase an SRI. The throttle response should be better than the CAI since there's a shorter distance for the air to travel, and no hydrolock to worry about.