The upper range is determined by the loudspeaker itself. THey will only start to help at the freq at which the loudspeaker is losing pattern control and becoming omni. At that point they will help to "redirect" the energy forward.

If large enough they can actually help the pattern control of a loudspeaker, although there will be a few issues with discontinunity between the actual horn of the cabinet and the wings, but they will help somewhat.