The OpenCL implementation provided by Apple is less than ideal. It lacks a number of optimizations that are provided by, for example, AMD's OpenCL driver packages for Linux and Windows.

That, coupled with the wretched GPU options available on the Mac Pro, make the current stack not really cost effective. (I think a 5770 is still $250.)

However, it is better than CPU-only.

I've found slaving a cheap Linux box with expensive GPUs to be a much more gratifying experience.