Here is what I do to get 28MHs+ on my MSI RX470 gaming X 4gb Hynix .  (Find out what type of chip you have in the GPU - You do different processes for each)

1) In ATIwinflash save your base bios:  Open atiwinflash and hit save.  Save the file. 

2) Strapping:  Open Polaris Bios Editor and on the timing (MHz) copy the 1500 data to the three settings larger than 1500. On Samsung you will copy the 1750 timing to the larger values.  Watch this video that shows you how to strap.  I did the same method:  https://youtu.be/OP_zSP2H6ho  This will get you 23.5mhs per gpu.

2) On the MSI Afterburner I set the Memory Clock to 2000 and the Core Clock to 1150(up to about 1275) and press the check(Apply).  This should get you the 28mhs.

3) Now I go to WattTool and look at the numbers for each gpu individually.  In this case for gpu "0:Radeon (TM) RX 470 Graphics" look at the mV number in "R7" = 1150mV.  I do not want this number less than 1000mV.  This is just my preference.  Look at the Voltage Offset.  Whatever number you put in here it will be multiplied x 6.25 so, 20 x 6.25 = 125.  1150 - 125 = 1025mV(which is above 1000)  so this one would be -20 and then click "Set"  This gpu should now produce much lower watts.

4) Go to the next gpu and evaluate the number and make the changes in the Voltage Offset based upon the mV number in "R7."  Each gpu will be different.

Good Luck

Let me know if you have more questions.