I am also having issues with this. Have fraps active, but not recording, and I get 160fps+ when SLi is maxed out. When I move around in the field the SLi indicator goes down and in some areas the SLi indicator shows nothing. When this happens my fps is at like 20 - 30. I have tried running fullscreen and borderless windowed and still get the same issue. My specs are as follows:

Intel i5 3570k OC 4.5ghz
12GB DDR 3 1600 memory
2x GeForce GTX 670 SC 4gb

I also have the game running on an SSD drive and using the newest beta drivers. This issue also was happening when I was using the newest WHQL drivers. I was getting about 16,500 in the benchmark so I'm sure that SLi is just not implemented properly yet. Just felt like giving a bit of information so the issue can be resolved ASAP.