I think you'll be fine. I use LX Monitor-Arm with a Cinema Display 27" (10kg) and it JUST ABOUT keeps it up in most positions. : ) If the iMac is about half the weight it should work great.

I use my Ergotron LX Monitor-Arm for over 2 years now with a Dell U3415W that is over 8kg. While the monitor arm works fine with the monitor, my table bends a little with the weight. But having a desk with the clear space under the monitor is really great.