It was pretty painless. I told the iMac to migrate all my settings, documents and applications over from my Mac Pro, and left them to it overnight. The next day, it all seemed to have happened.
I have selected the largest screen resolution available in the resolution menu, and set all the graphics options as high as they will go. The available screen res options seem to vary between App Store and Steam versions. There are no HDPI options listed under Steam, and at least one of the options that was listed resulted in a tiny window with miniature, unclickable buttons.
I don't really play the game for its own sake, so I always run it in a window. This allows me to flip between it and other activities. For example, I may be checking the forums, or running Xcode and other utilities if I'm working on code to analyse saves for GOTMs. I have tried running full screen mode, which scales well, and there may be other ways to set it up that I haven't tested yet.
As far as I recall, I was able to launch both versions of Civ5 after migration without any further action, and they both came up initially configured as they were on my Mac Pro. The only major change was that, as mentioned above, the App Store version now has its Mods menu enabled. The Steam version has since updated to the new patch.