I'm not sure as you are
closed platforms have always better performances, and sli is not a perfect multiplier
maybe 4 gpu (remember that 2 gpu are in a single board, they share memory access) will perform 2-3x the single card, is that enought to fill the gap with the closed, optimized platform, cell spu, memexport, core locking?
I'm not sure, maybe yes, maybe not
personally I think that a quad sli can run the game at higher resolutions, but at the HD resolution I really don't know if the pc will have the more stable performances