From Blizzard :
You can turn off shadows completely as well, this will help take out extra graphics as well.
Turn off shadows:
Go into the World of Warcraft => WTF ƒolder and open the Config.wtf file using TextEdit, then add the following line:
SET shadowlod "0"
In the game go to your Video settings and change the following sections:
Resolutions: have the Multisampling set to the lowest setting and you might want to
turn ON Reduce Input Lag
Now test the game.
Side Note: The human eye won't see a difference between 72 FPS and 100 FPS and over. To read a great article about this check out the following 3 part article:
http://www.daniele.ch/school/30vs60/30vs60_1.html
Also : With the game closed please go to Mac HD => Applications => World of Warcraft ƒolder and:
1. delete the CACHE ƒolder
2. move WTF and INTERFACE ƒolders to the desktop
Moving them to the desktop makes it easy to replace them if necessary.
Oh and
here is the Thread on Blizz forums which give you informations about performance for every Mac, included 2009.
For The Lazy :
Early 2009 iMac with 512MB ATI 4850 Graphics Card : 70-90 fps
Early 2009 iMac with 256MB or 512MB nVidia GT120 or GT130 Graphics Cards : 40-55 fps
Early 2009 iMac with nVidia 9400m Integrated Graphics : 20-30 fps
iMac with 256MB or 512MB nVidia 8800 GS Graphics Card : 60-70 fps