This in itself is very hard to do. Let me explain.
Due to the different natures of the P45 and boards with X38 / X48 chipsets it is almost impossible to have a completely even playing field. Heck even when testing boards WITH THE SAME CHIPSET there will be differences in gaming performance. I will draw your attention to a few articles:
Techreport's X48 Showdown (See ET: EQ benches)
Bit Tech's Multi GPU benches with the Gigabyte P45 DQ6
OC3D's P45 Showdown
Not only are there performance differences between X48 and X38 (basically the same chipset) when equipped with similar hardware but there are even VERY large differences between P45 boards when it comes to performance. We aren't talking about 1-2%, more like 10-20% difference in some cases.
So, the question begs to be asked: in a Crossfire vs Crossfire review between the X38 / X48 and the P45, how do we know that the performance (or lack thereof) of the motherboard itself and NOT the PCI-E bandwidth is being brought to the forefront? The simple answer is that there is really no way to know unless there are MASSIVE differences in performance.
Writing a review of Crossfire performance between two motherboards with different chipsets, getting results that are within 5-10% difference between the two products and then saying that there is a bottleneck is a stretch at best. That is why I look at some preformance results and take them with a grain of salt.
Will a P45 bottleneck a pair of HD4870 cards? Heck no.