Since this is a preview, we can't put out numbers yet. AMD has, however, made its own performance claims for the Radeon Pro Duo, tested on a typical high-end desktop (HEDT) machine.
At the outset, AMD claims the Radeon Pro Duo to be 1.5X faster than the GeForce GTX TITAN X and 1.3X faster than the outgoing Radeon R9 295X2. This becomes a recurring theme for the way the Pro Duo performs versus the two cards in popular games that can take advantage of AMD CrossFire.
AMD is also claiming the Radeon Pro Duo to be faster than two GTX TITAN X cards in SLI under "ideal conditions" (4K Ultra HD "Ashes of the Singularity," with DirectX 12 Async Compute, a feature NVIDIA chips currently lack).
The Radeon Pro Duo scores the highest in SteamVR performance assessment.