Regarding the problems with Yafray on OS X, I had the same problem. Luis helped me over in the Yafray forums; check his last post in this thread.

Quote:

* It takes ages! (I know I'll get flamed for saying that but it does) I sould mention that I was using GI a lot, The few renders I did without GI seemed fast enough.

I think GI is slow on any renderer; most GI effects can be faked with spotlights, which render much faster.