Yes, much better in Bootcamp. I picked up a new 27" iMac 2.93 GHz i7 Quad-core w/16GB RAM & ATI 5750 which you would think could run the Mac Civ V fine, but it doesn't. I get red specks early on and then later red & white checkerboard effects on the Mac side. Forget running a huge earth game.
Running in Bootcamp with Win7 64bit w/DX11 I've been up to 400+ turns on a standard map with no problems. It still bogs down on a huge earth in the later game. I run it at the highest resolution and screen size. On the Mac side, the sides are cut off and grayed out. There's some work that needs to be done to bring it to par.