I found a similar issue when using client-side api to change the empty message.
After some trying I found that the method of set_emptyMessage by itself will not update the empty message of radcombobox, but if you call clearSelection method on client-side, then the new empty message will show for the radcombobox.
I am using Q3 2015 version. May be a Telerik support professional can confirm this.
combo2.set_emptyMessage("Select a deleted Product to Recover");combo2.clearSelection();//this line is needed for new empty message to show up