My friend has one of the new mac mini and WoW is what I'd call barely playable on it. It will run around 15-20 FPS in instances, more like 5 FPS in a major city.

As far as the mac pro tower goes, it may work but you'd spend 6K$+ on that machine... it's overpriced and not worth it IMO. You can build a very good WoW machine for much less than 1000$, easily. So even building 5X of them you'd save 1K$+ without a doubt.