The regular ComboBox goes through all of the starting with the same letter items when pressing the same key multiple times. RadComboBox currently always highlights the first one.
In addition, the case where starting to type couple letters, will select the matched item with StartsWith

When Tab key is pressed inside of the drop down portion of the ComboBox the next control to be focused.

Available in the 2016 Q1 Release: Added IsDropDownTabNavigationEnabled property which provides an option to disable the Tab navigation in the dropdown of non editable ComboBox.