I would use GPU particle systems and create specific sized volumes of them, setup LODs on them based on the relative size of your volumes and place them in the level.
If your LODs are setup properly with LOD checktimes based on your movement speed and LOD method set to Automatic you should have very little performance issues with a simple translucent rain material using refraction and non-directional or fake lighting…emitting thousands of GPU rain sprites with depthBufferCollision set to kill.
If you use velocity aligned sprites with sizeBySpeed set to high values along Y Axis and low threshold you can get a really nice visual result.