Low hashrate on a RX 580 8gb

Would appreciate some assistance. Been mining RVN off of some RX 480 4gb and get an average of about 14 with modded bios. Was able to get my hands on a RX 580 8gb and decided to try stock setting and mine RVN. To my surprise, it was only pulling 12 Mh/s with stock bios. So I went ahead and modded the bios and was only able to pull 14Mh/s with OC. Is this normal? It has more memory so theoretically it should double the hash rate of my RX 480 cards right? This is with HiveOS BTW.