Muffinmix made a set of 'weather' particle emitters which would probably cause less lag than these. Every single projectile has an emitter on it as well as raycasts/collision code, compared to one brick emitter...
I prefer the emitters, if you don't want it to go trough bricks, let the bricks on top of those bricks emit them.
If i am right, this will stop the rain from coming in.
Also, the snow in the map:"The Slopes" goes trough bricks too..
