I was able to undervolt at -100mV, and then bumped up the power limit to +50% and got to higher clocks that way. But I eventually just flashed it with a Vega 64 BIOS with a 220w limit. Now it can hit 1380-1400Mhz without any manual tweaking. With the Vega 64 BIOS for some reason it became more sensitive to undervolting, and I can only get down to -40mV or so before it starts crashing. Undervolting it doesn’t seem to help a whole lot with the 64 BIOS.
My Vega 56 isn’t a top of the line card either. It has a blower style cooler, and I suspect it may have been used for mining. So take my results with a grain of salt…
If it is running higher clocks it will likely need more voltage to get there. So this kinda makes sense. Vega 56 limited to say 1330mhz will be power limited at the stock 1.2 (or is it 1.175, i forget) volts, but likely won’t need anywhere near that core voltage to get that clock speed. So you can undervolt it significantly.
Once you start getting higher clocks (e.g., with Vega 64 power limit), more volts are needed, but still the sweet spot with stock cooling appears to be somewhat less than the 1.2v that AMD pushes on the card stock - assuming your chip isn’t total trash binning.
edit:
my 2 are reference cards - cards with better cooling - YMMV.
On my sample size of two (both reference blowers) I’ve found 1.095v +50% offset to be the perfect spot to achieve 1532MHz turbo at 200W total power draw. This seems to put Vega 56 within margin of error of a GTX 1080 in a few games, and about 5% higher in canned benchmarks like 3DMark Firestrike. At +38% power limit it drops clocks down to 1447MHz but power also drops off to 181W.