Thanks for getting back to me so quickly!
After a little more testing on our own we decided to move away from using RadTreeView. We briefly considered implementing something using RadDiagram, as we have used that previously with great sucess. But in the end we decided to go back to basics and just implement it using standard Silverlight controls (Grids, Borders, Stackpanels, Textblocks etc).
It turned out pretty good. So we are happy with our current solution :)