Linux 4.18 AMDGPU Tests: Vega Taking A Hit
Dawn of War 3 also saw a significant drop in the RX Vega performance at 1080p.
ET: Legacy did see a sizable Vega performance drop at 4K, but then again this is a fairly non-demanding game with today's hardware.
F1 2017 was also struck by lower performance on Vega with Linux 4.18.
Mad Max in some scenes were trending lower on Vega with Linux 4.18.