I am trying to look that up. I am doubting myself now if it actually downclocks or not. The cards have to be normalized in output in order to have a smooth performance. So it doesn't make sense that one card will perform faster than the other in xfire.
If the faster card doesn't downclock, the end effect will act like a downclock where the 4870 would somehow get inhibited to the speed of the slower card.
Edit: Topper you are right, there isn't a downclock. I am still trying to tack down some info on what happens to the faster card.