IMHO - your biggest problem is that your drive will be bottlenecked in areas that load lots of content - e.g. Shat. (5 copies of wow have to hit the hard drive, scan a huge file to find the right sections to loadx5 - at the exact same time - lots of waiting and lagging).

If you could split 5 copies on wow on 2 different physical drives, you'd be set. I think that setup will run fine in most situations though.