Judging by the results, Sandybridge seems to improve minimum FPS, which the more I think about it the more logical it appears to be. At the moment I haven't seen much on tri SLI and Sandybridge but I'm willing to bet it performs well.
Thanks to this thread, I have just ordered another GTX 580, so I can try them in SLI mode (albeit in the 8x lanes for now, until I get the updated mainboard..)
Doesn't matter - all of my cards are running at 8x at the moment and it has made literally no difference to the performance. Indeed!