Regarding performance, try playing with loops and worksize (-z and -w), you can get a bit more out of it if you manage to find the optimum settings for your GPU. Check the the wiki and forum for that.
Also, regarding responsiveness, try adjusting the FPS (-f), at something like 60 or 120 you may loose a few MHashes but your UI experience will be smoother.
Some people also disable Quartz Extreme. That will make the window server render using the CPU instead of the GPU. That way you can regain good desktop responsiveness without losing MH/s. I haven't tried this myself but it should work. Since you are on an iMac, you should also get SMC fan control and boost your fan speed to keep the system cool.
I decided to just mine in Windows after having performance and heat issues in OSX and this is on a Mac Pro. The tools for monitoring and cooling the GPU are just better on Windows and Linux and Apple's OpenCL is seriously lacking. If you can use bootcamp to mine in Windows or Linux for when you don't need OSX, your computer will thank you. You could get Linux or even Windows to boot off a USB key, Bitcoin mining requires almost no HDD access.