I noticed a slight performance boost when I symlinked my wow folders.
However, I had patching issues twice and eventually needed a reinstall in both cases.

Case 1
Root WoW (nothing ran off of this).
Linked copies 1-6 (1-5 boxing options down, 6 raiding full graphics).

I patched the root wow, had some kind of an error. One of my clones would crash the game.
The other clones would get to a point where they wanted to patch but the game version was already good.
A GM eventually told me it was because I had previously run the beta on the system, despite uninstalling it at the end of beta.

Case 2
Same Set Up as Case 1.

Difference here, I broke the Symlinks prior to patching the main Root wow.
Had some stupid error while applying the patch.
Thereafter, got a red error message box at username/password screen, and could not progress.
Not positive if this was a SymLink issue or not.

Running 5 instances of warcraft, from a single install, on one hard drive... works fine.
I can run five clients around Dalaran; while not entirely smooth, I don't lose follow going from A to B at peak times.
I can battleground or raid with 40 toons near me fine.
Wintergrasp, like Dalaran was not entirely smooth but it is playable.

My system was pretty killer when I got it, but its "ok" now.
Q6600, 8 gb Patriot (Low Latency) ram @ 800mhz, HD 4870 X2, Raptor 10k rpm HD.
Nothing over clocked.

The difference between symlink and not is twofold for me:
a) Symlinked would be slow for a few seconds in Dal/Shatt, then it would be very slightly faster. Non SymLink stayed at the not faster speed.
b) Symlinked has required a reinstall every patch. Not Symlinked has never required a reinstall from patching.

For the very slight gain, imo - it is not worth the patching issues.




*Edit*
Regarding the multiple cores...

Warcraft has a few features for dual core processors.
With two cores running a warcraft the game is faster then one core running the game.

Keyclone had a recent thread for i7 affinity, something like:

Wow 1... Cores 2 & 3
Wow 2... Cores 3 & 4
Wow 3... Cores 4 & 5
Wow 4... Cores 5 & 6
Wow 5... Cores 6 & 7

That was not it exactly, because it used all of the cores, but there was overlap and 2 cores per warcraft client.