Yes we already do know this for sure. Read the P55 reviews, they're already suffering 5-10% hit when using CF/SLI setups vs the X58. 5870 will only use MORE PCIe bandwidth not less, so the hit while using 8x PCIe will only be higher.

X58 + 5870 CF is the best setup because that is the only way to provide 16x PCIe to both cards while having the fastest CPU to fully utilize both cards.

Quote Originally Posted by Astennu View Post
We dont know that for sure. We dont know what kind of bandwith a single HD5870 will need. And we dont know what AMD does to minimize the overhead.

What about CF / SLI setups that work on 8x config's. Like the P55 boards ? They should have the same problems then of not bigger.

We cant say anything about it untill the cards are out. Then we can do tests by overclocking the PCI-E bus so see if you get big performance gains. I think the PCI-E bandwith wont be a problem on 2560x1600. Especially with AA en AF. Then you can use all the horsepower you can get.