So, the first thing one needs to know is that 6 thunderbolt outputs does not means hook up six projectors and off you go.
The configuration matrix is a little strange but its based on the fact that there are three thunderbolt buses.
So after trying dual heads we ended up using apple dual link adapters, at the moment I have 4xDVI discreet stages and one thunderbolt for control and its stable.
In conclusion. Add 99 Dollars per extra dvi output you want from this machine.
Update: it seems like you can mix dual links and normal mini display adapters. I have 2+2 working now. The maximum of mini display port to DVI adapters is 2 like apple says, from there on you need the more expensive dual link adapters.