Loading and zoning (same thing really) will be lightning fast. Other then that; during normal gameplay/dungeons/raids/pvp I doubt you'd notice a huge difference. You'll still have to wait for the rest to load in

Fun, if somewhat expensive experiment, but I don't think it's worth the hassle. You'd have to copy over the WoW folder after every reboot. And let's face it, it's still Windows so it needs rebooting.