Sorry to bring up 8.1 again, but is anyone here actually able to play games on Windows 8.1?
If that's the case, could you please try MouseMovementRecorder and tell what you see?
You can get it here: http://donewmouseaccel.blogspot.se/2010/04/markc-mouse-acceleration-fix-builder.html
I'll be able to do it in an hour. I don't notice anything wrong at the moment and I'm sure this is going to break my little illusion.
IGNORANCE IS BLISS DAMMIT.