Well you shouldn't have expected 100 fps in Bf4 with the 270x to begin with, but oh well lol
And I'm assuming you have the Earthwatts or whatever they call that, and no the HCG? I wouldn't try pushing the 280x on that, as it only has 22A on 2 +12v rails. It just barely meets the minimum for the 280x, which I wouldn't recommend.