http://www.pasteall.org/pic/104303
If you look at the left half of the image you can see sharp lines in the dust. They are symmetrically placed on the right side and show through the pillars and benches. This is the only layer with this problem, the others work normally with z depth.
Also, for some reason rendering jumped to some insane numbers; last one threatened to be 24h, so I’m not keen on rerendering it.
Also, if I tried to render it on the same layer it came out differently than by compositing.
The blend file was too big so I had to erase a lot of data. I think I kept the important stuff.
http://www.pasteall.org/blend/42414