I am actually laughing my ass of right now

.
Vega in high level layout right now resembles very much Nvidia GPUs structure, however achieves higher throughput compared to previous versions of GCN slightly different way, than Nvidia GPU does. I know it may sound weird at first glance, however, this is best occasion for what I was planning for quite some time.
Vega's geometry, and graphics pipeline pretty much resembles right now Kepler, Maxwell, Pascal, and most likely - Volta GPU architectures. However, it is tied to Programmable Geometry pipeline, Pixel Engine and Primitive Shaders.
None of those features are present on previous versions of GCN. What this means is that if you have specific paths in games, and specific techniques of rendering which were tied with previous versions of GCN - that is what you will get - Fiji performance.
However, because you have similar features used in Vega, that are comparable to Nvidia GPUs, you do not have to do any specific thing to utilize them in your games. GCN has become pretty much Nvidia like, but has higher compute throughput, core for core, clock for clock, even higher than Pascal GP100, and Volta GPUs, which will resemble Pascal GP100 High-Level layout.
Dont worry, about Vega. It is very easy to rewrite the games to be compatible with Vega. You just add few marks in your engine for Vega architecture, and that is all.
Also, bare in mind this:
https://en.wikipedia.org/wiki/Feature_levels_in_Direct3D
Tiled resources. Compare GCN, and Maxwell, and Pascal. Now Vega is Tier 3.
Vega is supposedly FL12.1 highest Tier, for everything.