So as fast as a 6990 in OC mode... or not quite as fast as 2 6970's in crossfire.... Really, unless you have a space issue... Or too much money... 2 6970's are still better then this card... The lucid chip, in a bitmining context does absolutely nothing for you... Considering you can just launch extra miners, the lucid chip is only useful for making the two cards appear as one to games, and other big monolithic apps...
Save your money and get 6970s...
unless you're really hard up for space... but i think you could almost build a small box and 2 6970s for less then this behemoth...