What more RAM in your system brings to the table, is basically two things.
1) it enables more shared RAM to be made available to the video subsystem, which might improve performance a bit. It depends on how much video RAM is required for rendering and buffering of shaders, textures and what not.
2) it enables the Windows kernel to cache recent data read from, or written to the hard drive. The more RAM you have, the more data that can be cached like this. Note, caching data like this doesn't make the game any faster, unless it constantly would need to read data from disk (like Diablo IV does).
My system has 64 GB installed. And what the caching does is this: The first time I start Stellaris (haven't checked it with Victoria yet), the game takes about 2 to 3 minutes to load, since it needs to read a lot of files from disk. When I then quit the game, and restart it, it loads in less than 10 seconds. The difference between the first time and second time loading is all down to Windows caching all these game files Stellaris needs to load in the excess RAM my system has.
