It all depends on your memory type. I have 140+ RX580 running right now. The micron are the best with 33,5MH. SK Hynix and Samsung are between 30 - 32MH. i don't have Elpida.
Check your memory straps. You can try using amdmemtweak --REF 20 in the advanced config box, try 20 or 30. Some cards accept up to 240. wich will increase your hashes significantly.
You can also play with the core and memory state to lower your power consumption a bit.