Overclock3D did some digging and discovered a score of the Radeon RX 560X in the 3DMark Sky Diver benchmark database. This GPU is apparently used by an upcoming Acer laptop. The RX 560X appears to be an enhanced version of the RX 560M, with a Polaris 21 GPU with 1024 shaders, a clockspeed of 1275MHz, and GDDR5 memory.
So the gist of this is that the Radeon RX 500X will likely only be found in laptops. The new parts are expected to be a bit faster than the ones that were launched a year ago, but don't expect anything earth-shattering.