There's no such thing as "newer RX580 with dual memory". Each card only comes with memory chips of a single particular brand. For the most 580/8 these days it's either Hynix or Samsung (heard of cards with Micron, but never saw them myself). The manufacturers are just too lazy to bother with writing separate BIOSes for each memory type, so they make one BIOS with straps for 2 or more memory types in it. If the card has Hynix chips installed, it'll use the straps from the Hynix table, if it has Samsung chips — then it'll use the straps from the Samsung table etc. So when you edit your BIOS, you just edit the straps corresponding to the memory type installed on your particular card. Judging by your clocks and hashrates, you've got Hynix chips on your card.