So as of Friday some users already have been able to get a Radeon R9 390, as stated for a while now these are product as based on the 290 series, that's aside from the increase in VRAM and tweaks on GPU and memory. New photos of a user dismantling the product shows a XFX Radeon R9 390X graphics card taken apart to its PCB. That PCB is identical in design to AMD reference PCB for the Radeon R9 290X.
The PCB is so similar that down every SMT component and solder trace is similar. So the difference really is the usage of 512 Gbit GDDR5 chips and perhaps a different choke or capacitor here and there.
The Grenada GPU looks to be identical to Hawaii, the GPU used for the R9 290 series. This also means that the card is using GCN 1.1 architecture and that a lower DirectX feature level is supported (12.0). The images have been posted in the hardocp forums by sirplayalot, see source link.