Does using crossfire or sli do anything worthwhile for lunatics AP or MB?
Compared of course to 2 separate gpu's.

Nope. At least I've never seen any advantage to doing it and from what others have said, it doesn't seem to make the least bit of difference.

Those features are used to make your collection of GPUs work as one for rendering purposes. Where the slight lag between the devices talking to one another isn't very noticeable.
In the case of BOINC & SETI@home you are preforming a separate function on each GPU. Having a single task processed across several GPUs may cause a massive slow down in processing while data moves across the PCIe. Instead of internally on the GPU.

In the past I think you actually needed to disable those features in order for BOINC to use all of the GPUs. However with newer drivers I think it no longer matters.

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SETI@home classic workunits: 93,865 CPU time: 863,447 hours
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Thanks for the info. Just wanted to make certain I wasn't missing some opportunity.

I believe CUDA 6 adds support for using cards in SLI mode (ie treating them as a single larger GPU). It may take some coding and I don't think anyone has produced a CUDA 6 Multi Beam app (yet). Jason would probably be the person to ask.

XT Libraries: Automatic Scaling to Multiple GPUs

CUDA 6 introduces XT Library interfaces which provide automatic scaling of cuBLAS level 3 and 2D/3D cuFFT routines to 2 or more GPUs. This means that if you have one or more dual-GPU accelerator cards in your workstation or cluster node, you can automatically take advantage of them for intensive FFTs and matrix-matrix multiplication. cuBLAS XT also enables multiplication of matrices that are too large to fit in the memory of a single GPU, because it operates directly on matrices allocated in CPU memory, tiling the matrix and overlapping computation with memory transfers. The result is linear scaling of very large GEMM operations to multiple GPUs, as the following figure shows.

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BOINC blog