I recently crossfired a mis 7970ghz edition with a r9 280x asus brand. I have been getting frame drops in games. Full specs: amd fx 8350 asus formula z board 2x graphics cards and 16gb ddr3 memory 1080w psu (cooler power)
you have a good crossfire cable attached to both cards.
you may want to download msi afterburner to make sure that your gpu core and memory frequencies are the same. they need to be, i had that issue when I CF a 7870 tahiti le and a 7950