It would be nice to use a Grid instead of simple BoundList to render the ComboBox picker. This is currently not possible.

I currently use the following code to implement that feature. Maybe it is possible to integrate that into the standard ComboBox in future releases?

Code:
Ext.define('PVE.form.ComboGrid', {
    extend: 'Ext.form.ComboBox',
    requires: [
	'Ext.grid.Panel'
    ],
    alias: ['widget.PVE.form.ComboGrid'],

    // copied from ComboBox 
    createPicker: function() {
        var me = this,
        picker,
        menuCls = Ext.baseCSSPrefix + 'menu',
        opts = Ext.apply({
            selModel: {
                mode: me.multiSelect ? 'SIMPLE' : 'SINGLE'
            },
            floating: true,
            hidden: true,
            ownerCt: me.ownerCt,
            cls: me.el.up('.' + menuCls) ? menuCls : '',
            store: me.store,
            displayField: me.displayField,
            focusOnToFront: false,
            pageSize: me.pageSize
        }, me.listConfig, me.defaultListConfig);

	// NOTE: we simply use a grid panel
        //picker = me.picker = Ext.create('Ext.view.BoundList', opts);
	picker = me.picker = Ext.create('Ext.grid.Panel', opts);

	// hack: pass getNode() to the view
	picker.getNode = function() {
	    picker.getView().getNode(arguments);
	};

        me.mon(picker, {
            itemclick: me.onItemClick,
            refresh: me.onListRefresh,
            scope: me
        });

        me.mon(picker.getSelectionModel(), {
            selectionChange: me.onListSelectionChange,
            scope: me
        });

        return picker;
    }
});