For sure if you are wanting to reduce the 15GB installation for boxing purposes than symlink is one way to go. If you are trying to reduce the size of just one install than you have no real options.

I just symlink'd my 5 wow's. It was a different process for Win7 64bit . I used a program called HardLinkShellExt_X64. If you are symlinking you should take a look at it. There are other great post on this site for symlink if you want to look at options.