Sorry to tell you, but i doubt it. While the cards dont have to be the same AFAIK, as with regular SLI/Crossfire, there needs to be some performance parity. IIRC 280x is akin to 7970, that makes 270x 7870? And you paired it with 7570, which is pretty much low-end, so no wonder it made the game run worse.
I am not totally sure how it works with Ashes, but with regular multi-GPU set-ups, the second GPU renders the odd frames. Now imagine your 270x can do it at 30FPS, but 7570 only at 10...
