Artboard interpolation tool
Picture the scenario. You have an art board, you copy the art board and completely alter the spacing. So now you have one art board that has a tight fitting UI, and one that has a spacious one (too spacious!)
You might play around with it for a while until you find a good point somewhere between the two art boards. But these minute changes can be difficult to keep track of.
My solution would be for art board interpolation. You select both art-boards and right click to interpolate them. This opens a screen with a simple slider that lets you interpolate between your tight fitting UI and your looser one. If you drag to the left, it becomes more like your first art-board. If you drag to the right, it becomes like the second. This would allow you to more quickly experiment with different spacing. I think FontLab does something similar somewhere by allowing you to slide between two different fonts.
It could be useful to speed up UI design.