Over the weekend some more details about the Radeon R9 390 series leaked, including PCB shots that confirm that the PCB of the Radeon R9 390X is identical to the Radeon R9 290X. The GPU codename is Grenada but it looks like it's pretty much a rebadge of the Hawaii chip that drove the R9 290 series.
GPU-Z screenshots are making the rounds and these confirm the Radeon R9 390 lineup will be little more than rebrands, with more memory and faster clockspeeds:
Unlike older, more blatant re-brands, such as GeForce 8800 GT to 9800 GT, AMD did drop in a few changes. To begin with, the memory amount has been doubled on both cards, to 8 GB. The memory clock has been increased from 1250 MHz (5.00 GHz GDDR5-effective), to 1500 MHz (6.00 GDDR5-effective), resulting in memory bandwidth increase to 384 GB/s, up from 320 GB/s. The core clock speed on the R9 390X is 1050 MHz (up from 1000 MHz on R9 290X); and 1000 MHz on the R9 390 (up from 947 MHz on the R9 290).