By separate installs I meant that only the data (maybe cache and interface) folders are symlinked. The rest of the full WoW directory structure is indeed separately installed. I ran 5 WoWs on 3 executables if you want to call it that.

Others may have more input on this part, but I'm pretty positive that reading it from RAM for the other 4 WoW instances via symlinking will still be noticeably faster than having the 4 instances read the data all over again from an SSD. I don't think that matters if they are all from the same install directory/executable or if they each have their own install/executable. The data still needs to be read by each instance.