With multi-threading turned off, it seems it still gets split across the cores in the same percentage. However I had found that not only would it not visually update the render until it was done, but it had a good chance of crashing as well.
I should also note that I'm using Windows 7. Such behavior may be different in older operating systems.
Do you guys remember that tool someone made to increase rendering times of single frames by splitting it in parts by covering a portion of the camera's view? Not sure how effective it'd be in ART, but it did work for the scanline rendering, I believe. Another idea is to render a still frame normally in one instance, and then flip the camera upside down and render it in another instance, and then halfway print+screen and splice together...though that'd only work for images less than the screen resolution.