You can play 16xx res, with max settings{i7} (though I am not sure if you were applying the "Extra" option or "High")
Here is one difference though, I am running the 1.5 version, and you most likely are on 1.7, the patch difference includes some GPU optimizations especially for AA and stuff.
So that could account for some of it.
All 3 textures on "Extra" and with the patch installed. I've tried 2xAA on at least one res setting and it nearly halved the frame rate.