I doubt it. It will cost you $300-$400 to replace the two 275's with DX11 cards that are comparable in speed. I don't know the differences between DX10 and DX11, other than better support for GPU programming (OPENCL) and a change in how tesselation is done.
Will this make a difference in BF3? Will EA actually create different rendering code for DX10 and DX11 that exploits the new features in a way that is noiceable? I highly doubt it. DX10 is already very powerful for the types of effects that games use, and the actual look of the game will depend more on the techniques they use for things like shadows, particle systems, reflections, and other effects than any differences between DX10 and 11. Since most of their games are just console ports, I don't see them maxing the capabilities of DX10.