Right now, I'm regularly over 100fps, usually pegged at 120fps in most of my games, using a 144hz 2K monitor connected via Display Port. So with my intent of moving to macOS, I expect the same results now that I can run a desktop GPU. Apple went with AMD, so G-Sync is out.
Hence the need for full support of FreeSync.
As for my next comment, I don't intend for this to be insulting or talking down to you; some may not understand why there is a need for such high frame rates.
It's not that one needs to be running at 120fps. It's that you need a system capable of delivering at least 30fps, if not 60fps at the most intense times. The horsepower involved in hitting that mark thus results in hitting such high framerates when things are easier to render for the GPU. Think of it as a cushion, headroom, or a reserve if you will. FreeSync and G-Sync tie the monitor's refresh rate to the outputted frame rate from the GPU, thereby removing tearing where the the monitor can't keep up with the GPU.
So yes, if I can't cant connect a proper display with the proper cable, the enclosure is useless. There are other enclosures that you can pop a 580 into and have access to all of the ports that the card actually supports. Like Display Port.