After applying the .reg and after rebooting you must go in wattman and click reset. Then click manual voltage settings and then click apply. Next to got HBCC slider. If it's on, turn off, turn on again, slide to the max and apply.
Also note line 3, where I have 0001 is my board number as seen by Windows. Your Windows might have assigned a different number so you have to change it accordingly.
Detailed info and source here
http://www.overclock.net/t/1633446/preliminary-view-of-amd-vega-bios/250#post_26297003Figured it out. Had to apply the 900mV to all P-States starting from P2. Not just only P4 to P7.