Okay, I'll redo it with the transparency set to zero - I thought this was required, oops.

edit: yeah I can already see that it's behaving quite differently this way, it seems to be making many many more ray hits (red dots).  This is a good sign, to me.  Render should be finished inf a few minutes.

edit: I can see this is killing polygon smoothing (not really surprised, but disappointed).

My Freebies