I made this AddOn to improve work in cycles on the viewport.
This tool automatically adjusts the render border, significantly improving render speed.
PLAY / STOP: This option automatically adjust the border every refreshing blender.
The limitation of this tool is the number of vertices in the scene, you can slow blender!
MANUAL SET: This option is suitable for large scenes. Sets the value of a click.
MARGIN: It sets a margin value for selected objects.
The tool works with selected objects.
If you do not select any object tool work on all objects on the screen.
This tool will be improved over time, but it can be used.
WARNING: This tool does not work for sequences render! for now
BLENDER ARTISTS POST: http://blenderartists.org/forum/showthread.php?388043-AddOn-Automatic-Render-Border