So, we are waiting for a GCN1 driver.
GCN2 and GCN3 were skipped by OS4 in favor of Polaris (GCN4).
While GCN5, Vega, is already available and what may be the last generation, GCN6, Navi will be out this year.

Is this a correct summary?

Considering that GCN architecture is completely unrelated to Terascale architecture...

I am again conflicted.
I would be easier for Mark to complete 3D support for the remaining Terascale cards, than to struggle to learn a new family of cards that will soon have 6 generations of development.

And my benchmarks of the HD6870 vs the HD7850 did nothing to convince me that there was a lot of improvement between these two architectures. After all, we don't utilize GPGPU computation, which is where GCN shines.

Still on the basis of availability, and the pricing of lower end GCN4 cards, maybe the jump makes sense.

RX560 cards are little more than $100, and RX570 cards like the one Mark purchased for his PC start at about $130.

Alternately, used Radeon HD6870 cards can be had on eBay for as little as $22.

I have no firm opinion on any of this.

I'd like a GCN1 driver, a GCN4 driver, but would probably get just as much performance out of a cheap HD6870.