With my dual GTX580's in SLI:

Resolution: 5760x1200
General Draw Quality: 8
Everything else: Max
AO & DoF: off

It's not that the game hates ATI cards. It's that they lack the balls that nVidia has.

Find your video card on this list:
http://www.videocardbenchmark.net/high_end_gpus.html

Then compare it to the cards performance that you see people comparing high settings to, and perhaps you will see why you are only playing at medium settings. And the curve is exponential, low to medium may take +10 video card power, but medium to high may take +20 video card power.