Just the "normal" in-game setting and vanilla SMAA, on a 670. Higher AA settings or straight up downsampling directly gave me too many noticeable dips into the 50s, mostly when driving in the day, although it's probably worth it for the IQ =P
Edit: Although, then you still get stuff like this (down from 1440) http://www.abload.de/img/hkship2012-08-2209-17jnuwf.png so maybe not, especially if the only frame drops come while you're driving, when you're not going to be noticing small IQ improvements anyway because of the motion blur.
I have a 670 and can downsample from 1440p with 60fps at night even in the rain, but any daytime scenes get me 57-58fps at most. I'm guessing it's the shadows. Same with AA set to High too (at 1080p). 60 at night, 55 during the day.