It should be, but that's a 4 rail design. It's possible to max the power out on certain rails, and still not be at the rated capacity of the PSU. I found this out with my Strider 850, when I burnt it out with 2x GTX285's on the same rail. My guess is that you might be running into the same problem. This is why I prefer single-rail designs.
Edit: Here's JonnyGuru's review of that PSU.
Antec CP-850 850W Review