At the recent dog and pony show we all noticed the new Vega GPU playing a game at 3820x2160.
The AMD Vega would need maybe 64 GCN cores to be able to achieve that? give or take a bin.
That would be enough to play most titles at 3820x2160
My HD 7870 has 20 GCN cores which beats consoles which is what it was procured to do. No game is unplayable.
The RX 480 has 36 GCN cores which is not enough in my opinion for 3820x2160 but it should be able to handle the most demanding games at 1920x1080 without even breaking a sweat.
Vega may have a new core that is different, but AMD has been successful with the GCN so this is the comparison I am using.