Fake Fullscreen doesn't seem to work if I remember right. I played with that a little bit trying to address the same problem. I'll re-try it after work to see if it does, indeed fix the issue.
I have multi-monitor too, its been working great for me. You can also turn on edgelocking so the mouse doesnt escape and you have to alt-tab to leave it. Im using Vulkan though, DX11 didn't like that at all haha. Vsync off but thats about it. Hopefully it fixes it!
