Remember, the 285 has 12.5% less shaders and TMUs compared to the 7970/280X. Also, you have to remember that the 285 clocks like the 7970, about 10% lower than the 7970Ghz/280X and R9 380X.
With all this in mind, I would expect the R9 380X to have a good margin over the R9 280X since both are clocked similarly. Here, we have the Sapphire 285 at 965mhz boost, the 7970 at 925mhz and stock 285 at 918mhz. Keep in mind that the 7970 maintains this speed all the time while the sapphire 285 won;t keep 965 all the time.
The 285 manages to match the 7970 despite fewer shaders/tmus due to the improvements from GCN1.2. I would expect the 380X to have a good showing over the 280X, probably in the 10% range.
That is not to say that getting 2 used 7970 for CFX won't be worth it. Just saying, that this cards makes sense as a 280X replacement. For someone who only buys new, the 380X will be a better choice as 280X stocks dwindle, ofc if AMD prices the card right. It does target the gap between the 906 and 970 right now.Warning: Spoiler! (Click to show)
Edited by LuckyStarV - 10/23/15 at 12:25pm