DisplayContent is used to represent the descriptor visually in the GridViewGroupPanel, not in the group header.
What you can do is use the GroupHeaderTemplate property of RadGridView and bind to the group key with the StringFormat you need. The DataContext for the GroupHeaderTemplate is an instance of the GroupViewModel
class. In your case, GroupHeaderTemplate can be a single TextBlock, whose Text property has a binding with path "GroupKey" and whatever StringFormat you need.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items