I think you will find that with the Slats deployed all or most of the hot air is vented and will not de-ice the leading edge. That is why its use is recommmended prior to extending that flaps/slats on approach, hence the reason for the note in the FCOM that extended flight in icing conditions with flaps extended `should be avoided`!
Last edited by Meikleour; 27th Feb 2010 at 19:00.
Reason: addition