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).