Are you talking about using AMD CPUs or GPUs or both.
I'm using an AMD FX-8320 CPU with a nVidia 660ti GPU and WoW works fine in D3D9 mode under Ubuntu 13.04. I get about 45-55 FPS on ultra settings, sometimes it drops down to 30-ish FPS but it's still playable.
There isn't really anything you need to do on the CPU end if you're using a AMD chip. Should just run with it. On the other hand a GPUs performance is bound by their driver. What ever GPU (AMD or nVidia) you are using make sure you are using the proprietary driver and not the open source one. Maybe the open source driver became reactivated and it's reverted back to using it. The Proprietary drive will definitely yield better performance, so double check you are using it.