Cheers Piltdown Man,
I wonder if the real answer is "it depends on the type of LE device, and the angle of attack at the time the device is extended." Or perhaps it's as simple as "you're right"!
Relevant accident reports - NTSB/AAR-93/07 (effect on lift unspecified), NTSB-AAR-81-8 (deployed slats resulted in decrease in lift) and perhaps some others I couldn't find.
Cheers,
O8