I don't think I've noticed that big difference in LFS and I'm on GTX 295 as well.
I think I currently have my GTX set to run with single GPU permanantly, but even with dual GPU turned on I have enough FPS to not to worry about it. Could be that your issue lies elsewhere. I'll can check again when I get back home (7ish hours from now), because I'm not 100% sure if I remember correctly.