It's really hard to see what the section box is going to cut when you adjust its sides. We often have to readjust it multiple times to get it right. Why not activate a transparent blue plane when you move it. That way, you would clearly see where the box is going to cut the model.
This idea is similar to the dynamic section box idea but it's hopefully faster to implement and will not require a high end machine to use it.
Also, it would be helpful if the blue planes would highlight on mouse over of the section box icons. That way it would be immediately clear that you are on the sweet spot.