Fullscreen stretch seems to be drawing a bit of my start menu (docked to left side of screen) and some desktop widgets on top of the game: http://i.imgur.com/5nSnq.jpg
Fullscreen borders makes the game lock up and need killing with task manager...
I also notice it uses more GPU horsepower than Diablo 3?
Edit: By forcing vsync I have reduced the GPU load on my SLI 580's from 98% to 12% However it now takes about a minute to load, rather than a couple of seconds...