The cards will always run at thier set speed. Unlike SLI there is no synchronizing of the cards clocks.
The performance will still be equal to two of the slowest card (they are given the same amount of work, the frame is donw when teh slowest of the two is done). However, if the faster card is teh primary for all applications that do not use crossfire you will get the full overclocked performance.
Though it is easy enough to OC the other card, though the black eddition xfx "might" be faster than some other 5870's could reach.