Well, SLI is just straight data through put, it is parallel processing. If it were distributed processing I could see it being way better. I think that multi-core GPUs will replace SLI very soon.
Even then, your average photoshop usage will not take advantage of your over clock. I think it is a niche market and only appeals to those who are hobbyists and like the horse power and like benchmarks and maybe like pushing their systems to the limits.
Otherwise, over clocking offers no real time or real world benefit for any average user or average professional user. I mean I occasionally use photoshop, illustrator and indesign, but none of my work is that complicated and anything I may be doing wouldn't benefit from it.
Also, over clocking makes it unstable, which is why you see render cards and xeon processors in real work-horse workstations. A Quadro card sucks bad for gaming, but is awesome for Maya or 3DsMax. Because it is designed to be slow and stable and crunch the numbers, where as a gaming card is meant to render polygons and calculate physics on the fly. Different animals, different user oriented goals.
(makes it quite heavy to run tough,.. sadly)
