As others said, you must do different things - optimize BIOS settings, change frequencies, voltages and configurations. In the first step you should use the new AMD beta driver for blockchain(at AMD page) then use MSI-Afterburner to find out the best frequencies for GPU and MEM. You can change the values during claymore is running and see the changes in hashrate directly. Generally start by reducing GPU-clock and increase the MEM clock. Generally start the optimization work with only one card in the rig !
Another suggestion - use W10 64bit. You can expect at the end up to 31,5 Mh/s with RX580. When you change the BIOS later keep in mind you need tools for editing and flashing BIOS. And you need a driver patcher ... you can find everything by search. Good luck