I've been looking for head-to-head comparisons of P67 vs X58 GTX 580's in SLI within the same post, but so far I only came across the following two posts. I cannot imagine there's such a difference, so either I'm seeing things or there's something apparent with x8/x8 vs x16/x16 in high-end SLI testing 3DMark Vantage Performance setting. I was expecting maybe 10-15% difference and not out of the ball park.
Ideally, you want the same exact enviornment. Clearly, on a single GPU my pick is the P67, but SLI/CF on high-end GPUs my feeling have been leaning towards the X58 until I know there's no difference. I've seen saturation head-to-head with GTX 470's and resolutions at or above 2560 X 1600 showing the X58 ahead.
Interesting saturaturation charts. P67 28,800 vs X58 46,656 or are my eye's all blurry?!