If power is not an issue you should better go with liquid cooling and run dual @ 300 watt.
300watt is to much for reference air cooling.
But if you pay for electricity then air cooling is ok, even rx vega 56, just run at 130-160watt and forget about dual-mining.
I have no vega 56 yet but I see no problems in flashing it with vega 64 bios and apply proper tuning then.