I'm going to take a look a this, but I feel there maybe several changes coming to the ComboBox.
I'm really not happy with it's current state and there are several shortcomings that need to be resolved. The ComboBox has turned into a very challenging control to build and it's going to get a major overhaul in an upcoming release.
Adding the OnItemChanged event is fairly trivial and can be done quickly, but I'm worried that the current functionality is not properly implemented. Things just don't sit right with me at the moment and I need to work out some of these concerns before adding more functionality that might cause breaking changes further down the road if we determine things were not implement properly again.