Anyway, TechPowerUp loaded up the new Crimson ReLive 17.9.2 drivers and decided to check if the CrossFire performance scaling is as good as AMD promised. Due to the very limited availability of the Vega GPUs, TechPowerUp put a Radeon RX Vega 56 and a Radeon RX Vega 64 in the test system. They simply do not have two of each so they had to do a test with mixed cards.
You can view the results over here but it's clear that mixed Radeon RX Vega CrossFire makes zero sense at the moment. The site tested nineteen games and found CrossFire delivered meaningful performance increases in just four titles. In almost all other titles TechPowerUp tested, the performance gain was minimal or even negative:
Enabling CrossFire with GPUs that are based off the same die and family but with a different shader count has been something AMD has done for a few generations now, and we continue to appreciate this remaining an option. However, this is no excuse for the performance we are seeing here, be it a driver issue or otherwise. As can be seen from the chart above, a lot of games not only show poor scaling relative to a single RX Vega 64 but actually negative scaling.
Furthermore, the site encountered issues with visual artifacts and display corruption in a couple of games so it looks like the Radeon RX Vega CrossFire drivers are still buggy. Hopefully, someone can test two Vega 64 or Vega 56 cards soon.