I have the Arctic Twin Turbo 2s in Crossfire for my Powercolor AX6850s
I'm running an overclock now of 850/1200 and it is stable in 3dmark11 thus far. The memory clock is a little higher than usual, but I don't recall that ever working before I installed this cooler. 1150 is still good though if I happen to encounter instability in games.
Temperature hits 70C in Far Cry 3, highest I've hit yet. The second card is a good 10C cooler. VRM (on 1st card) are a little warm but safe in the 70's. The setup pulls in air from the rear and sides of the case, and exhausts it vertically. My H80 in push/pull has no trouble exhausting the heat set on high.
I have a Gigabyte 990FXA UD3.
http://ecx.images-amazon.com/images/I/61HqP7rb6SL._SL500_AA300_.jpg
It's a squeeze in a 7 slot case, but they do fit if your mobo has that kind of setup. Luckily my seventh slot is on top or else I'd be using USB for wifi.