RE: downhaul specific
This will depend a lot on the specific sail you use. Some sails keep behaving well when downhaul is eased off so that there is no visually distinguishable free leech but others do not work in this setting. What can happen is that the sail completely loose its range and keep working, but only in a super narrow range of (light) wind. What can also happen is that the head gets so tight that it sort of brakes the sail and don't let it flow thought the air even in light wind.
It is hard to identify which sails that are gonna behave in a certain way wrt this. For example, I remember that my 2004 Hot SO took well to a tight leech while the 05 version did not, despite them looking virtually identical.
Overall (and this applies to outhaul too) the only way to learn is to experiment. Start with a "safe" setting and then adjust outhaul or downhaul and try to feel what happens.