I have a new iMac i7 with the better GPU. This replaced a 2011 iMac that was also max-ed out.
CIV V plays better on the new machine with graphic options set to the mid-range. On the old machine, I could count on the terrain become a red and white checkerboard after the game got to about the mid-point, and progressively worse as I went on. It still did this on the new machine, but to nowhere near the same extent.
I have both the Steam and Apple store versions, and had the same issues with both. (I bought the App Store version when it came out, hoping that it would resolve the problem, in fact. Obviously, it didn't.)
That said, the game is still very playable IMO on an iMac. And, as was said above, there is always the dreaded Boot Camp option.
Now if I could only find Civ 2 somewhere for OS X.