Are you getting the D300 or D500?
I got the D700. In single GPU, Open GL kind of performance (e.g. Uniengine Valley benchmark), it performs the same as the 680MX in my iMac when on native 27" panel res and ultra/4xaa. Within a few percentage points. That was surprising. (That's good but not great.)
Go to Windows where you get Crossfire and two cards in use and it doubles. Or using OpenCL under Mac it can double too.
For Mac gaming, it's nothing special. But Apple chose these chips for compute; AMD is stronger under OpenCL than Nvidia (though Nvidia is vice versa under CUDA vs OpenCL.)