The RX 580 will do ~315 H/s on ZEC. The Claymore epoch benchmark figures are just an estimate and largley overstated. In my own test with a five card RX 480/580 8GB and 570 4GB rig. The benchmark at epoch 129 for the five cards was ~143.3 MH/s, at epoch 130 the benchmark was ~142.5 MH/s. The actual current dual mining hash is ~144.1 MH/s dual mining with DCR. Even a 10% drop in the RX cards ETH hash in 6 months can be offset by addding a single $200 RX 570 4 GB card.
Where does the huge difference on ZEC mining coming from between 1070 and rx580?
At ETH a GTX 1070 has similar MH/s than the RX580 but at ZEC it has 50% more (~ 480 Sol/s). From gaming perspective they are also very similar from performance.
And yes, this is a serious question as i'm interested in technical details due to my work in IT business

For the mining, not the most powerful GPU is the best, cause GTX 1070 for gaming is madness, and for mining its not so good as its price