Here's an article on CF between P45 and X48.
Intel P45 Vs. X48 Crossfire Performance
Author: Shane Baxtor
SUMMARY: We take the time to analyze the difference between the x8 / x8 configuration on the P45 and the x16 / x16 on the X48...
Here's the same setup but with Foxconn P35A which had PCIe 1.1 16x and 4x slots.
You can see the difference. :) Having two PCIe 2.0 16x is much better don't you think? :)