The other guys are saying to run of a single install. The generic SavedVariables is now per Account, not install. You can check your WTF folder to see that. Now, with multiple clients running you are still using the same Data files so you are not taking the performance hit.

I ran two clients this way yesterday and was fine. Addons were were fine and my performance was normal.

All that said, if you are still running symlinked, how are you dealing with the locks on the MPQ files?