Launched yesterday by AMD was the Radeon R9 285 "Tonga" graphics card, a GPU that's derived from their Tahiti GPU core. The Radeon R9 285 has a $249 price point and comes in 2GB and 4GB GDDR5 versions. Unfortunately we weren't seeded with any Radeon R9 285 so don't know how well this new Rx 200 series graphics card works under Linux, but they released a Catalyst update that appears to support the new hardware for Linux users.
AMD put out the "Catalyst Software Suite for AMD Radeon R9 285" for Windows and Linux. While it's aimed for the R9 285, the driver will work with any supported Radeon GPU. Besides enabling the R9 285 Tonga, there's a bump in the OpenGL support so hopefully it will end up working out better for Linux gamers wishing to play the latest titles on Steam. I'll be putting this new AMD Linux driver through its paces later today on some other GPUs.
In terms of how well the Radeon R9 285 works with the open-source RadeonSI Gallium3D driver, I have no clue at this stage and haven't seen any chatter of anyone attempting such. Seeing as Tonga is derived from Tahiti, it probably should work like the other Rx 200 series GPUs on the open-source Linux driver -- assuming the PCI IDs are present. We'll probably have a clearer picture if the R9 285 works on the open-source AMD driver in the days ahead.
The new Catalyst Linux update can be found from this support page.