I wanted to send this as a PM but I need at least 15 posts before I can do that...
It looks like you can modify just about anything about the effects in the specialeffects.tec file. It looks like you can also set different effects for different surfaces like gravel, sand, grass, brick...
I don't know if modifying it will cause issues online though.
I decrypted it, you can download it from:
specialfx.zip - Race 07 effects file - Dedicated Web Hosting Service (since I see another mod that uses this file on RSC I assume it's ok for me to post this?)
Note to everyone: this download is only of use to a modder, it's not going to change anything in your game as it is.
It goes in "\race 07\GameData\Locations".
Maybe you will find that useful
Just hope it works online..
There is the "stunning rainfx" mod on RSC so if that works online without issues then this will too, since it's based on the same file.