Shadows on high and decorations on were a bit of a performance hit but nothing huge. If you were going for optimal performance while still leaving shadows on in some way I would just set them to low.
The main thing to remember is when you are hiding in that dark corner waiting for someone to walk by you might be completely exposed depending on other peoples video settings.
I
asked about some of the other settings on the official forums and got a few more pieces to the puzzle. I'm going to try to get some more screenshots later.