I tried many things and after some time I gave up on trying to make both the editor and the game work with particle collision. So, I changed everything from 30 to 5 and the particles from 5000 to 500. Then I placed many rain particles all over the map except the room covered with planes. It looks just great and I'm getting 60fps(again, with so many particles I need to test this with lower-end hardware when the mod is near-release). So my recommendation is using many smaller rain particles since collisions do not work properly. Thank you Statyk for all your help/research though!
Btw, some lite fog added. This map is now done I think. It has changed so much lately and from what it was in the beginning! Even the recent moddb photo is now completely outdated.