Recently I was able to get Mantle running on Win7 with a homemade SDK. But discovered neither OpenCL nor Mantle (nor OpenGL) provide enough information to resolve which GPU (eg Bus, Dev, Fnt) is associated with which Windows PCI Enumeration Registry entry. Not a big deal, but it would be nice to have an API response that uniquely identifies each device.
Having taken an in-depth look at OpenCL and Mantle, my personal preference is Mantle. It would also be nice if AMD would compile a mantle32.dll set that works on version 5 Windows OS's. The future occurrence of Vulkan probably not withstanding, Mantle is sufficient and close enough to the register programming guides.
On another subject, I read that AMD is planning on delivering a new CPU in 2016. To put a competitive edge on the next CPU iteration, I would like to suggest a variation on the double down R9 295x2 philosophy: leap the toad stool. AMD should skip over DDR4 support and instead push ahead with DDR5 RAM for either 64 GB or 128 GB on the motherboard.