To elaborate on some comments above:
If you mismatch cards in SLI (in terms of the clocks designated in their bios), they will *apparently* run at two different speeds ... meaning that if you check the graphs in Afterburner, each will be running at the speed designated by it's bios.
However, you will likely find that the higher-clocked card will always have a slightly lower GPU Usage % vs the lower-clocked card, as it will be 'waiting around' for the slower card to finish it's rendering task.
The net result will be that the *actual* performance will be the same as it would be if both cards were at the lower of the two clock rates.
The fix for this is to use Afterburner to set them both to the same speed ... very easily done