Just to drop a number: With current C31 code Vega 64 does 1.1 g/s at 180 W (settings: 1380 / 1000 @ 0.912V) making it the 2nd most profitable algorithm after CNR for V64 (!!!).
You are totally wrong with your calculation. Vega64 can easily make 50+mhs eth with lower than 180w power consumption. ))