I'm in a situation where I can exchange the RX 590 for a Vega 56 but I'm having a hard time justifying it from a productivity perspective.
I'm satisfied that a Vega 56 would improve gaming performance at a resolution of 2560x1440, but it's unclear whether it will be "better" than an RX 590 for Final Cut Pro editing, and, if so, exactly how. At the moment, there is not much in the way of objective testing about the real world productivity benefits of an external GPU, let alone as between an RX 580 or 590 and a Vega 56 or 64. It becomes a question of how much one is prepared to spend to experiment.
This video by Max Yuryev is sobering:
In Activity Monitor, one can open up a window dedicated to showing what the GPU is doing (⌘4). It's helpful, but all it tells me its whether the current task in the application is GPU intensive.