x8/x8 crossfire worth it?

I am looking at buying this motherboard:

http://www.asus.com/Motherboards/M5A99X_EVO_R20/

My question is if I get a Gigabyte HD 7950 and decide too add another later for a crossfire setup will be they be significantly bottlenecked by the fact that the motherboard only supports pci-e 2 x8 when using 2 GPUs? I understand that there is the M5A99FX that has dual x16 capabilities but my store doesn't stock it :(. Would it be worth paying $75 more for a motherboard that supports x16/x16 (The Sabertooth 990FX R2) or is x8/x8 enough to use without much performance decrease.

Thank you :)

P.S. I have nothing against other brand motherboards but Asus are the only brand in my shop that have the USB 3 internal connector required for my case.

Just buy the Asus M5A99FX PRO R2.0 its like $10 more for CF and SLI @ x16 x16 dont buy from a shop order online you buy at a premium in store.

I have seen benchmarks before that Crossfire x16 x16 performs only a tiny bit better than x8 x8, there is only a 2-5 fps gain. The big leap in performance is PCI E 2.1 to 3.0, sadly there is only one AM3+ board from ASUS that supports it and along with being rare, it is also a lot more expensive. As well as, the current AMD FX CPUs don't really have solid support for PCI E 3.0..... If you google for the benchmarks, I believe Tom's Hardware, as much as I hate using them for reference, did tests on this.

I agree with cooperman, none the less, get the 99FX board from ASUS for $10 more, as you will gain a lot more benefits, such as a better power phase design and better quality parts for overclocking, etc.

Thank you both, I have found a new and cheaper store that sells it, now I have some extra cash to put into my GPU