that is exactly what I thought .
when trying to allocate cores via a bat file, the wow instances always "fell back" to core 1 & 2, so I assumed Blizz forced wow on 2 cores for a reason.

Now I run my master instance (all visuals to the max) on cores 1 & 2
and the 4 slaves on cores 3 & 4

(even though 1 instance on each core, with instance 4 and 5 sharing worked well in outlands too)