I remember when the TrashCan first came out, that people complained the D300/500/700s were already a generation behind, but Mac stalwarts (never Apple themselves) predicted that newer cards would be on the way from AMD as the Trashcan grew in popularity. Never. Happened. (Neither the popularity or the cards)
Get the feeling nobody was interested in making fanless PCIe versions of their card with an SSD slot shoved into the middle of it along with a normal compact version in the form factor Apple wanted. Since the cards never changed, AMD probably didn't want to waste any developer time or effort on bringing more advanced cards to the Trashcan.
I'm glad that AMD and Apple have been working together to tighten up the driver code so that what I do on GPU tasks is possible, but still am annoyed at them for refusing to acknowledge that nVidia exists.
Devoting 1 CPU and 1 GPU to one Einstein task takes 18 minutes to complete on the RX 580 machine, and 20 minutes on the D700 machine, with the RX 380 machine only behind both by a few at 26 minutes.