Good question. I haven't an answer though lol.
My setup is
drive c: Pally install and Warlock install - as I play them solo here and there
drive e:1 install for 2 mages and 1 priest as I never play them solo

Last night I junctioned just the data folder on the Pally and lock to the alt folder on drive e:
The performance was pretty good, much better than running 3 different data directories, however I would like to know if there's any benefit performance wise with linking other dirs.