I also have one Sapphire Vega56 with Samsung (not flashed, stock bios) that I tested with. Core/Mem settings: 1407/975 @ 905mV.
I have 3 of those, for ETH mining, push -REF 65500.
=> 46 MH/S with much lower clock and much lower power (140 to 150W@wall).
Could you share you core/mem clocks and the whole rig power? Are the bioses flashed to 64?