This is the 1st time I've seen this benchmark so I'm not sure what kind of a score you would get with (2) 5870's in Crossfire. I would expect a big boost but Crossfire usually doesn't scale as well as SLI.
Also, while I ran the test with DX11 selected, my hardware doesn't support all the DX11 features. You may get a higher score by running the Benchmark in DX10 mode which is all my system is actually capable of.