Borderless Window here... (GTX 690)

I have two monitors with the main display running at 2500x1600 and then a 2nd simple older LCD 21" display for video/music/web/*gasp*work*gasp*.

There is a performance hit in FPS with the 2nd monitor enabled, but it's minor. (~128 FPS in idle/clear area and ~60/50 FPS in Mega S-Rank Hunt event)