I'd have to try a demo or something. I'm still running a first gen i7 and a 5870. I still haven't put a new system together, since of course I still haven't found anything that could really justify it. 15 FPS is pretty unacceptable. Have you been adjusting settings? Does it allow for fine manual adjustments or does it have a simplistic "Good, Better, Ultra" joke of a graphics menu? Can you try dialing back the antialiasing and setting the V sync?
Unless of course they've got such shit network and optimization code that the connection lag is actually causing graphical lag.