TLDR version: Symlinking saves space. I cannot see it improving performance for the reasons listed below.
Maybe I misread things, but a lot of posts on these boards implied significant performance boost when symlinking. One of the ways this would be evident would be what I tested. Apparently that's not the case. Aside from about 3-4 months, I've played wow since f&f beta pretty hardcore, so I have a pretty strong sense of 'typical' and 'atypical' performance.

First, if there's a way to objectively test and quantify results for the improvement in load times between all five clients, let me know and I'd love to test that and share the results.

I would think though, that if symlinking improves the seek and load time of 5 instances of wow because it's only having to find the file once, then I would guess that one would only see a relatively minor increase in load times, not a linear (or worse) increase. Additionally, if the clients are indeed sharing information, then it makes little sense for them to load 5 separate copies of the same files. Here's why:

There's two scenarios. Either 1) The WoW client is managing the file io and loading textures/etc into memory or 2) The WoW client passes the request on to the OS, which manages the file io and loading into memory.

1) In the first case, the WoW client would know the seek location for that bit of data and either share it with the other clients, or not. 1a) If it shared the info (which I doubt bc of security), then there's little rational for it to not point to the memory location of the textures instead of the physical drive. 1b) If it did not share this info with other clients, then each client is indeed seeking and locating its own data independently of one another.

2) In the second case, the OS is aware of the seek location and managing the passing of memory on to the wow client. The OS would likely handle requests by loading a textture, etc into ram and then when the same requests comes in from client #2, it would provide the same handle to the client, not reload the file and provide another pointer.

If neither the client nor the OS provide any sort of advantageous pointer/cache sharing, then symlinking has virtually no effect. If you're aware of the measures Blizzard takes to protect the reading of data from their client, it makes a lot of sense that the wow clients do not share data between one another.

I would love to be wrong about this so that our hardware doesn't have to scale linearly to maintain performance. If someone has ideas for objective tests that can be repeated and quantified, let me know.