hi,
I have 6 rx 470 nitro+ 8 Gb and all my card can do -96 mv.
For XMR 600 W for 4500 h/s in sgminer-gm so your result are not so great.
100h/card more than you isint great? My cards do -125mV, and its a really easy thing to mod in the bios, OP has stock voltage, surely you must understand what that entails. Editing the timings by hand, not that easy.