I don't know if this might be of use to anyone, but I came across this app by chance the other day and installed it on Windows 10.

It can do borderless full screen with downsampling, among other things.

Add pCARS64.exe to the list and if you want to use the downsampling mode, add "+" in front of the exe. Then add the following launch command to pCARS in Steam...
-windowsize x,y (where x,y is the resolution it tells you to configure).