after several months, the issue continue present on linux 3.16
Next on my wishlist is better fan management for the 6870. It seems that the catalyst doesn't have this issue, but the stock AMD fan is very noisy, and the card is kept at 69°C in a well-cooled case (motherboard at 32°C).
An other user reported the issue on the phoronix forums.
But I guess that's a very low priority item, and I should go with a custom cooler instead.
Great work, AMD devs, thank you