When IsEditable=true RadComboBox displays one of the telerik:TextSearch.TextPath, DisplayMemberPath or ToString() (in that order). If you are using ItemTemplate you could customize the displayed value in the TextBox by setting telerik:TextSearch.TextPath. Unfortunately there is no support for Converter when getting the property value. Instead, if your scenario allows this, you could override the ToString() method of the data object and return the proper value.
Here is a simple example, demonstrating the most common usages of the properties above:
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