Yes it probably would. Manufacturers like to tweak clock speeds on cards. For instance, reference GTX boards are supposed to run at 430mhz. However, most manufacturers will clock them anywhere from 430mhz up to 490mhz. Same goes for memory clock speeds.
When you run two boards with slightly different clock speeds you create a bottleneck. Same principle applies to system RAM.
The most optimal way to run SLI is with 2 identical cards. But as the people above me said, it is not required. Only same model is required.
Not long ago it was required that cards be identical for this very reason: disparity between clock speeds. The new nVidia forceware drivers fix this by adjusting clock speeds to match on both boards.