Borderless windowed mode is actually not as high as your native resolution. If you have 1920x1080 fullscreen normally, you would have to open the ffxiv config file, manually put in 1920 and 1080 for the resolution, and the set the file as read only under properties when you finish so the game doesn't default it back to the lower value. I only know this because I use sweetfx for better AA and had to do this for borderless windowed to work properly with it.