It appears that you can indeed increase the performance without much hassle by undervolting the RX Vega. There are other nice side-effects of this: the card does not get quite so warm and therefore fan does not make as much noise either. By raising the power consumption limiter, you can then put the saved energy to good use for extracting the higher clocking speeds. The effective speed gain at the optimal settings is 8% higher than on the default.
The question remains: why does AMD allow this free performance gain, seeing as it is as easy for us as it is for the home users to undervolt the RX Vega cards? If AMD supplied them with the voltage set lower, the RX Vegas would be faster straight out of the box.
We cannot think of a different answer than it depends on the percentage of the RX Vega cards that are not usable at the lower than 1,2V voltage that are still out there. However, for most of the RX Vega users, it is advisable to try out what is possible with undervolting.