I'll double check tonight but from my testing the VDDC GPU and memory settings are separate as long as the values are manually set in Wattman.So, core of this thing is an undervolted champ, running just peachy at 1000 MHz @ 0.8 V. ram, not so much. Hard crashed at 2180 MHz@ 1 V, artifacting at 2150 MHz @ 0.965 V.
Per GPU-Z this chip is garbage, bring better than only 7% of samples. Maybe that translates to terrible overclocking but the opposite when you underclock.
GPU-Z appears to report GPU core VDDC incorrectly (reports memory VDDC instead of GPU).
I'll check with my killawatt to ensure this is true though and let you know.
Also don't let the ASIC quality concern you. My lower ASIC quality cards use the least amount of energy. My worst card is the highest one at 90%.
Make sure you reboot after Wattman changes. Wattman is buggy. I haven't seen a reference 480 not hit at least 2180 on memory. You could have a dud or more likely Wattman is being stupid.
Also try Claymore miner in addition to Genoil.