I tend to find A to B track mods a bit boring and too ambitious for their own good. Now, this one is amazing. The trackside detail, the ditches, the cliffsides, trees, scenery, everything is so well-crafted and varied. If this were somehow Kunos original content, it would fit right in quality-wise. "It doesn't feel like a mod" is the best compliment I can give. It's amazing what this game can morph into in the right hands.
Does RainFX work on other tracks for you? Do you have another file for the track in '\Steam\steamapps\common\assettocorsa\extension\config\tracks\loaded' maybe? It definitely works on my setup.