We took a preliminary look at mining performance from AMD's new Radeon RX Vega 56 and 64 cards and found they performed a little better than Polaris parts, though nowhere near the 70-100 MH/s claims (in Ethereum) that were rumored prior to launch. That's the good news. The bad news is that mining performance may improve over time, perhaps even significantly, as AMD tunes its drivers for blockchain compute.
This is something that is already happening. After making an effort to help Radeon RX Vega cards land in the hands of gamers by bundling them with special "Packs" at higher price tags, AMD has gone and quietly released a beta Crimson ReLive driver update that is optimized for blockchain compute workloads.
Fortunately for gamers, the driver update still does not push mining performance to crazy high levels. However, it does improve things by addressing a lingering problem—decreasing hashrate performance with Ethereum mining as the DAG file size slowly increases. This is something that affects both Polaris and Vega.
Nathan Kirsch at LegitReviews tested the new driver on a handful of graphics cards and saw decent gains in some cases. You can view them here.
This beta driver release essentially fixes a problem that affects AMD GPUs more than Nvidia GPUs. It is not a game changer, though the writing on the wall is that miners are not going to be ignored by AMD and Nvidia. Neither side indicated plans to do so in the first place, but with optimized drivers now starting to show up, the volatile cryptocurrency market is further validated.