Re: SLI/MultiGPU drop FPS
It's possible that those benchmarks aren't really suitable for testing your machine. Which benchmarks stress gpu over cpu etc varies a fair bit from machine to machine.
I have tested ET and saw same crazy results as you. I have also tested Unigine Tropics, and I had strange results. Sometimes results would vary a lot, and so I have come to the conclusion it is too heavy a test for my machine.
Can I suggest that you try Lightsmark 2008 ? It's also available in the phoronix suite. I have found it gives a good balance to test graphics, including SLI, for me.