Interesting. I have a similar setup (R9 270X, i5 3470, 8GB DDR3) and I'm also getting extremely bad fps. Only 1-5 fps everywhere, even on LOW. It didn't seem like I could disable AA in the game completely, though? Minimum setting is 1.
Meanwhile I can play the game on my laptop with a geforce GTX 960M with only minor slowdowns in busy areas...