Hi bird,
Thanks for your interest in Syncfusion products.
To achieve textbox like filtering in combobox type filter in griddataboundgrid, custom filterbar has to be created with some more code to support this.
Please refer to the sample in the following link which shows griddataboundgrid wired with custom filterbar of comboboxtype supporting textbox-like filtering functionality(by typing in cell) to some extent.
http://www.syncfusion.com/support/user/uploads/I52165custom_f432b163.zipRegards,
Rajadurai