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.