I run 5 computers and have 3 Macs, 2 PC's.

I will say that WoW runs faster on PC's than Macs. Although the software is still "native" on both, it seems that Blizzard has taken much more time optimizing code for PC than Mac. It may just as well be that Macs run OpenGL whereas PC's still run DirectX and that difference is what is responsible for the relatively slower performance on Macs. I'm not talking just the iMac either; I have a Mac Pro 8 core tower that doesn't seem to run as well as my Dell 4 core PC.

If you are wanting to run multiple instances of WoW on one Mac, my advice is to get multiple drives and use CloneKeys. Not sure if it is still up to date, but it worked fairly well on a Macbook Pro 15" (model released just prior to the unibody MBP). Copy the WoW folder on each drive and then make shortcuts or Dock icons to launch each copy of WoW. Running on separate drives should help somewhat with performance.

If you have multiple Macs and want to hardware 'box, I use a keyboard multiplexer from Aten that is USB based. In retrospect, I would recommend a model from Vetra, as others have had good results from using their products and they do not require special cables. I think you can probably use a PS/2 based broadcaster in combination with PS/2 to USB convertors. Some people have had issues with USB broadcasters, and although I have had some issues with stuck keys and so forth, generally my setup has been reliable.