No, it would downclock the fastest card.
I've used 4890s of different speeds (much less different cards all together) and clocks were downclocked to the lowest clocked card. I dont know if they've fixed that since the 9.7 drivers but that was my experience.
I have a 4850 and a 4870 and have run them both in x-fire, and it never downclocked the 4870 to the 4850. Once my son has finished on his comp, i'll re-install the 4870 + 4850 in x-fire if need be and post screenies.
If I recall correctly (the 4850 has 512MB 4870 1024MB) it never dropped the 4870 to 512 either, but don't quote me on that, not 100% till I try it.
Here is an old screen shot of the 4870+4850 in x-fire.
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