I have more test for you, I try your 4K footage with 2X R9 290X crossfired. I am able to play back full best frame rate with 3 Sony FX applied. When I try to add the 4th Fx the frame rate drop back down to 21xxx at best/Full. I did not try to disable crossfired. But I will try that at later on. But right now, Crossfired enable work very good.

When I was run a single R9 290X I was only able to play back best/full with 2 Sony FX applied.