Have the Nitro+ RX570 too, but with 4GB - with Hynix RAM.
Used the stripe from bughatti or Ubermix 3.1 (the same) from 1625 up and reduced voltage to 900mV. Depending on your miner used you get different hashrates. The best rate I got actually with the newest Claymore 9.8 version is 29,8Mh/s - by using the new AMD beta driver(called Radeon Crimson relive Edition Beta for [Suspicious link removed]pute) on Windows 10. With Ethos(Linux based miner) I got only 26,6Mh/s. Another parameter impacting the hashrate is the RAM-type - Elpida slower, Hynix fast, Samsung fastest.
Optimize GPU and Mem-Clock - I got the best results with: GPU - 1170 Mhz / MEM - 2080 Mhz. Check the best values for you with MSI-Afterburner. Keep in mind that you need the atik-mdagpatcher, if your BIOS is changed or the new AMD-Beta-Driver is used.
Hope that helps to get better rates out.