The performance hit is much larger than 5-10% from what I've read. Basically from the numbers I've seen you should forget about crossfire with a second slot at x4.
Sad but true!
I learned this the hard way, bought a 970 mobo for my 8350.(always take the time to do your homework, there is no rush!)
Single gpu will do better on 970 board vs CF
I had to take out my second gpu 7950 to be able to use the first one to its 100% and with the CF it was all choppy. that was just false advertisement I say, it shouldn't say crossfire on the box even!