Hi,
there has been a similar discussion about Metal for Cycles on the mailing list. Here’s a link: [Bf-committers] Metal GPU support.
In summary, this is what Ton said:
[…]
But if you want to help us with Metal:Cycles has been designed from the ground up to support multiple graphics
APIs.
It has good Cuda and OpenCL support for that reason. Here it’s possible to
add Metal as well. Cycles code base is much smaller and much better
designed (and newerthan Blender code.
Why not volunteer first to help with Metal for Cycles? If that project
looks like it works well, we (and you!) also get more insight in the
complexity of having this kind of work done for Blender. Adding Metal in
Cycles will take 3-6 months of work already.
[…]