This is an amazing week - not only did AMD help break up Cycles into smaller kernels, but now they've taken a big step towards proper support on OS X too!
Ton Roosendaal writes:
After having struggled with limited OpenCL support for years, the OS X driver team at AMD made a big step forward by supporting function calls in the OpenCL compiler now. This is in the Beta 3 release of El Capitan.
That means that Blender now can support the current Cycles render engine for Mac Pro (10.11) with similar GPU render quality as Cycles had for CUDA in the past years.
Note: the "split kernel" version of Cycles stays in development, this would be the preferred way to have Cycles work in the long term. That work isn't finished yet though. For as now, a "mega kernel" would (and is on OSX 10.11 now) outperforming the "split kernel" in both quality and speed.