Apparently, some custom Vega cards suffer from issues with incorrect power and clock values if you use the 17.11.3 and older drivers. So if you're not running version 17.11.4 on custom Vega cards, there's the possibility that you won't be able to unlock the full performance potential.
Several vendors like PowerColor and Gigabyte have presented custom-design Radeon RX Vega video cards earlier this week. The PowerColor card is expected to be available next week, but we don't know in what sort of quantity.