Some clarification from Next Limit:
Maxwell GPU rendering is not currently supported on macOS. For GPU rendering, Nvidia graphics cards with CUDA support are required, and Apple does not sell computers which include these at this time.
If you do have an older Mac model with an Nvidia card, we cannot guarantee you a great experience, because those cards are obsolete and have not been tested.
Should Apple include graphics cards appropriate for GPU rendering in the future, we will likely support these. We are also studying the possibility of supporting third party Thunderbolt GPU interfaces, which may provide a viable GPU option for our valued Mac customers.
Also, I think it is important to note that Next Limit has been working on GPU integration for several years. This is an extremely challenging technical work, especially as their goal is to provide an engine equal to the CPU engine, not a limited alternative.