Actually the Graphics Test 1 of the new 3DMark FireStrike supports SLI all the way to 4 cards in a pretty linear way. That by itself can be a simple and quick way to benchmark.
Also I tested Metro2033 using its official benchmark tool, up to 3 Titans it scales almost perfectly, especially at high resolution:
...and fps contributed per Titan. See how consistent it is @ 4K resolution. Every Titan you throw in works equally hard!
Problem is games are not free. People won't buy a game just to participate our benchmark competition...