I had the same problem here is function that I wrote to select a string from the comboxes' that use comboLbox. What it does is opens the dropdown listbox control in the combobox and sets the cursor in the listbox. This causes editbox portion of the combobox to update allowing you to read the string and compare it.
#Include <GuiComboBox.au3>
#Include <GuiEdit.au3>
#Include <GuiListBox.au3>
Func ComboLBox_SelectString($Window,$WindowText,$ControlID, $SelectString, $fPartialOK = True)
Local $tInfo
$returnVal=False
$hCombo=ControlGetHandle($Window, $WindowText,$ControlID)
If _GUICtrlComboBox_GetComboBoxInfo($hCombo, $tInfo) Then
$hEdit=DllStructGetData($tInfo, "hEdit")
$hList=DllStructGetData($tInfo, "hList")
$EditTxt=_GUICtrlEdit_GetText($hEdit)
If (((StringInStr($EditTxt, $SelectString)>0) AND $fPartialOK) Or((StringCompare($EditTxt, $SelectString)=0) AND NOT $fPartialOK)) Then
$returnVal=True
Else
;Open ListBox Control
_GUICtrlComboBox_ShowDropDown($hCombo, True)
;Find SetText and Select
$i=0
$ListEnd=_GUICtrlListBox_GetCount($hList) - 1
While($i<=$ListEnd AND NOT $returnVal)
_GUICtrlListBox_SetCurSel($hList,$i)
$EditTxt=_GUICtrlEdit_GetText($hEdit)
If (((StringInStr($EditTxt, $SelectString)>0) AND $fPartialOK) Or((StringCompare($EditTxt, $SelectString)=0) AND NOT $fPartialOK)) Then
$returnVal=True
_GUICtrlListBox_ClickItem($hList,$i)
EndIf
$i=$i+1
WEnd
_GUICtrlComboBox_ShowDropDown($hCombo, False)
EndIf
EndIf
Return $returnVal
EndFunc