since there is no way atm to restrict how to mod your game, no this is not illegal.
also, since its just rain that's not a problem because it doesn't give u an unfair advantage over other players. I personally play with particles off because of fps issues as well.
what would really be frowned upon would be wall hacking (transparent walls), removing trees leaves and bushes and changing textures on weapons in order to make it easier to see them. again not illegal but most definitely frowned upon.
