Really? Damn, that's a bummer. I already ordered an SLI motherboard to replace my Crossfire motherboard.
I assume you are running the latest drivers 256.35? As far as I know, Linux shares 95% of their Nvidia drivers with Windows, shouldn't the performance be comparative? What did the devs have to say about this in your thread?
How did you enable SLI? Which xorg option did you use and can you paste your xorg here?